RESUMEN
The purpose of this review is to critically examine phosphene induction and saccadic eye movement generation by electrical microstimulation of striate cortex (area V1) in humans and monkeys. The following issues are addressed: 1) Properties of electrical stimulation as they pertain to the activation of V1 elements; 2) the induction of phosphenes in sighted and blind human subjects elicited by electrical stimulation using various stimulation parameters and electrode types; 3) the induction of phosphenes with electrical microstimulation of V1 in monkeys; 4) the generation of saccadic eye movements with electrical microstimulation of V1 in monkeys; and 5) the tasks involved for the development of a cortical visual prosthesis for the blind. In this review it is concluded that electrical microstimulation of area V1 in trained monkeys can be used to accelerate the development of an effective prosthetic device for the blind.
Asunto(s)
Fosfenos/fisiología , Movimientos Sacádicos/fisiología , Corteza Visual/fisiología , Animales , Estimulación Eléctrica , Electrodos , Redes Neurales de la Computación , Neuronas/fisiología , Neuronas/efectos de la radiación , Fosfenos/efectos de la radiación , Psicofísica/métodos , Tiempo de Reacción , Movimientos Sacádicos/efectos de la radiación , Umbral Sensorial , Corteza Visual/citología , Corteza Visual/efectos de la radiaciónRESUMEN
Two major cortical streams are involved in the generation of visually guided saccadic eye movements: the anterior and the posterior. The anterior stream from the frontal and medial eye fields has direct access to brainstem oculomotor centers. The posterior stream from the occipital cortices reaches brainstem oculomotor centers through the superior colliculus. The parietal cortex interconnects with both streams. Our findings suggest that the posterior stream plays an unique role in the execution of rapid, short-latency eye movements called 'express saccades'. Both the anterior and posterior streams play a role in the selection of targets to which saccades are to be generated, but do so in different ways. Areas V1, V2 and LIP contribute to decisions involved in where to look as well as where not to look. In addition, area LIP is involved in decisions about how long to maintain fixation prior to the execution of a saccade. Area V4 does not appear to be directly involved in eye-movement generation. In the anterior stream, the frontal eye fields, and to a lesser extent the medial eye fields, are involved in the correct execution of saccades subsequent to decisions made about where to look and where not to look.
Asunto(s)
Encéfalo/fisiología , Movimientos Oculares/fisiología , Animales , Mapeo Encefálico , Estimulación Eléctrica , Estimulación Luminosa , Movimientos Sacádicos/fisiologíaRESUMEN
The receptive field, defined as the spatiotemporal selectivity of neurons to sensory stimuli, is central to our understanding of the neuronal mechanisms of perception. However, despite the fact that eye movements are critical during normal vision, the influence of eye movements on the structure of receptive fields has never been characterized. Here, we map the receptive fields of macaque area V4 neurons during saccadic eye movements and find that receptive fields are remarkably dynamic. Specifically, before the initiation of a saccadic eye movement, receptive fields shrink and shift towards the saccade target. These spatiotemporal dynamics may enhance information processing of relevant stimuli during the scanning of a visual scene, thereby assisting the selection of saccade targets and accelerating the analysis of the visual scene during free viewing.
Asunto(s)
Corteza Cerebral/fisiología , Neuronas/fisiología , Movimientos Sacádicos/fisiología , Campos Visuales , Animales , Cinética , Macaca mulattaRESUMEN
This study examined the effects of anterior arcuate and dorsomedial frontal cortex lesions on visually guided eye movements in three rhesus monkeys. Lesions of the anterior bank of the arcuate, where the frontal eye fields reside, produced major deficits in the execution of saccadic eye movements to sequentially presented targets that did not recover even after 1 year after the lesions. Ablations of the dorsomedial frontal cortex, wherein the medial eye fields reside, produced much smaller and shorter-duration deficits on this task. Deficits after paired lesions of the anterior arcuate and dorsomedial frontal cortex were of approximately the same magnitude as after single anterior arcuate lesions. Anterior arcuate lesions also increased saccadic reaction times to single visual targets and decreased saccadic velocities that recovered gradually over a period of 2-5 months. Dorsomedial frontal cortex lesions produced only small increases in saccadic latencies that recovered rapidly. None of the lesions produced deficits in executing combined saccadic and pursuit eye movements to moving targets. The results suggest that the anterior arcuate area plays a central role in the execution of sequences of eye movements to successively appearing targets.
Asunto(s)
Movimientos Oculares/fisiología , Lóbulo Frontal/fisiología , Animales , Sensibilidad de Contraste/fisiología , Macaca mulatta , Estimulación Luminosa/métodos , Desempeño Psicomotor/fisiología , Tiempo de Reacción/fisiología , Movimientos Sacádicos/fisiología , Factores de TiempoRESUMEN
This study examined the effects of anterior arcuate and dorsomedial frontal cortex lesions on the execution of saccadic eye movements made to paired and multiple targets in rhesus monkeys. Identical paired targets were presented with various temporal asynchronies to determine the temporal offset required to yield equal probability choices to either target. In the intact animal equal probability choices were typically obtained when the targets appeared simultaneously. After unilateral anterior arcuate lesions a major shift arose in the temporal offset required to obtain equal probability choices for paired targets that necessitated presenting the target in the hemifield contralateral to the lesion more than 100 ms prior to the target in the ipsilateral hemifield. This deficit was still pronounced 1 year after the lesion. Dorsomedial frontal cortex lesions produced much smaller but significant shifts in target selection that recovered more rapidly. Paired lesions produced deficits similar to those observed with anterior arcuate lesions alone. Major deficits were also observed on a multiple target temporal discrimination task after anterior arcuate but not after dorsomedial frontal cortex lesions. These results suggest that the frontal eye fields that reside in anterior bank of the arcuate sulcus play an important role in temporal processing and in target selection. Dorsomedial frontal cortex, that contains the medial eye fields, plays a much less important role in the execution of these tasks.
Asunto(s)
Lóbulo Frontal/fisiología , Movimientos Sacádicos/fisiología , Animales , Conducta de Elección , Percepción de Color/fisiología , Sensibilidad de Contraste/fisiología , Discriminación en Psicología/fisiología , Iluminación , Macaca mulatta , Estimulación Luminosa/métodos , Desempeño Psicomotor/fisiología , Tiempo de Reacción/fisiología , Factores de Tiempo , Percepción del Tiempo/fisiologíaRESUMEN
Two eye fields have been identified in the frontal lobes of primates: one is situated dorsomedially within the frontal cortex and will be referred to as the eye field within the dorsomedial frontal cortex (DMFC); the other resides dorsolaterally within the frontal cortex and is commonly referred to as the frontal eye field (FEF). This review documents the similarities and differences between these eye fields. Although the DMFC and FEF are both active during the execution of saccadic and smooth pursuit eye movements, the FEF is more dedicated to these functions. Lesions of DMFC minimally affect the production of most types of saccadic eye movements and have no effect on the execution of smooth pursuit eye movements. In contrast, lesions of the FEF produce deficits in generating saccades to briefly presented targets, in the production of saccades to two or more sequentially presented targets, in the selection of simultaneously presented targets, and in the execution of smooth pursuit eye movements. For the most part, these deficits are prevalent in both monkeys and humans. Single-unit recording experiments have shown that the DMFC contains neurons that mediate both limb and eye movements, whereas the FEF seems to be involved in the execution of eye movements only. Imaging experiments conducted on humans have corroborated these findings. A feature that distinguishes the DMFC from the FEF is that the DMFC contains a somatotopic map with eyes represented rostrally and hindlimbs represented caudally; the FEF has no such topography. Furthermore, experiments have revealed that the DMFC tends to contain a craniotopic (i.e., head-centered) code for the execution of saccadic eye movements, whereas the FEF contains a retinotopic (i.e., eye-centered) code for the elicitation of saccades. Imaging and unit recording data suggest that the DMFC is more involved in the learning of new tasks than is the FEF. Also with continued training on behavioural tasks the responsivity of the DMFC tends to drop. Accordingly, the DMFC is more involved in learning operations whereas the FEF is more specialized for the execution of saccadic and smooth pursuit eye movements.
Asunto(s)
Lóbulo Frontal/fisiología , Primates/fisiología , Campos Visuales/fisiología , Animales , Electrofisiología , Lóbulo Frontal/anatomía & histología , HumanosRESUMEN
We assessed the effects of varying the time at which electrical stimulation was delivered to the dorsomedial frontal cortex (DMFC) and the frontal eye fields (FEF) relative to the onset of a visual target. Monkeys were required to fixate the visual target to obtain a drop of apple juice as reward. We found that the probability of eliciting saccades increased with increases in the delay of electrical stimulation relative to target onset. Also, the current threshold to evoke saccades decreased as electrical stimulation was delivered later following target onset. There were major differences in the magnitude of this effect with stimulation of the DMFC versus the FEF. The current threshold to evoke saccades from the DMFC was 16 times greater when electrical stimulation was delivered 200 ms after target onset as compared to when it was delayed 200 ms after target offset. In contrast, the current threshold to evoke saccades from the FEFs was only three times greater when stimulation was delivered under similar conditions. These results suggest that the FEF are more closely connected with the saccade generator for the execution of saccadic eye movements than is the DMFC, even though both regions have direct projections to brainstem oculomotor centres.
Asunto(s)
Conducta Animal/fisiología , Lóbulo Frontal/fisiología , Movimientos Sacádicos/fisiología , Animales , Estimulación Eléctrica , Fijación Ocular/fisiología , Macaca mulatta , Factores de TiempoRESUMEN
When monkeys are presented simultaneously with multiple stimuli, they can make one of two types of response. Either they make averaging saccades, that land at intermediate locations between the targets, or target-directed saccades, that land close to one of the targets. The two types of saccades occur at different latencies and are thought to reflect different processes; fast reflexive averaging and slower target selection. We investigated the latency of averaging saccades in five monkeys, with particular emphasis on 'express' latency saccades, which are thought to be inhibited by target selection. Express averaging saccades were made prolifically by the two monkeys that made both express and regular latency saccades, but only when no specific instruction was given regarding the saccade target. When these monkeys had to choose one of the targets, on the basis of its color, they still made averaging saccades. However, the endpoints formed two distributions close to the targets as opposed to one single distribution centered between the targets, as was the case when targets were identical; also, express saccades were almost entirely absent. We conclude that express averaging saccades are a form of spatial and temporal optimization of gaze shifting.
Asunto(s)
Macaca mulatta/fisiología , Movimientos Sacádicos/fisiología , Animales , Estimulación Luminosa/métodos , Tiempo de ReacciónRESUMEN
In normal vision, shifts of attention are usually followed by saccadic eye movements. Neurons in extrastriate area V4 are modulated by focal attention when eye movements are withheld, but they also respond in advance of visually guided saccadic eye movements. We have examined the visual selectivity of saccade-related responses of area V4 neurons in monkeys making delayed eye movements to receptive field stimuli of varying orientation. This task did not require the monkey to attend to orientation per se but merely to foveate the receptive field stimulus. We present evidence that the presaccadic enhancement exhibited by V4 neurons, quite separate from the response at stimulus onset, is a resurgent visual representation that seems as selective as the response is when the stimulus first appears. The presaccadic enhancement appears to provide a strengthening of a decaying featural representation immediately before an eye movement is directed to visual targets. We suggest that this reactivation provides a mechanism by which a clear perception of the saccade goal can be maintained during the execution of the saccade, perhaps for the purpose of establishing continuity across eye movements.
Asunto(s)
Movimientos Sacádicos/fisiología , Percepción Visual/fisiología , Animales , Macaca mulatta , Neuronas/fisiología , Tiempo de Reacción , Corteza Visual/citología , Corteza Visual/fisiologíaRESUMEN
The amplitude and direction of saccadic eye movements evoked electrically from the dorsomedial frontal cortex (DMFC) of monkeys vary with starting eye position. This observation has been used to argue that the DMFC codes saccadic eye movements in head-centered coordinates. Whether the amplitude and direction of the evoked saccades are also affected by changes in head position has never been demonstrated. Such a result would argue against a head-centered representation, and instead would suggest a representation anchored to another body part. Tests were conducted on rhesus monkeys to determine whether changing the position of the head with respect to the trunk or changing the position of the head with respect to the gravitational axis alters saccadic parameters. The amplitude and direction of saccadic eye movements remained invariant to such manipulations. These findings confirm the claim that the DMFC encodes saccadic eye movements in head-centered coordinates.
Asunto(s)
Lóbulo Frontal/fisiología , Cabeza/fisiología , Desempeño Psicomotor/fisiología , Movimientos Sacádicos/fisiología , Animales , Estimulación Eléctrica , Macaca mulatta , Corteza Motora/fisiología , Nervio Oculomotor/fisiologíaRESUMEN
In the frontal lobe of primates, two areas play a role in visually guided eye movements: the frontal eye fields (FEF) and the medial eye fields (MEF) in dorsomedial frontal cortex. Previously, FEF lesions have revealed only mild deficits in saccadic eye movements that recovered rapidly. Deficits in eye movements after MEF ablation have not been shown. We report the effects of ablating these areas singly or in combination, using tests in which animals were trained to make saccadic eye movements to paired or multiple targets presented at various temporal asynchronies. FEF lesions produced large and long-lasting deficits on both tasks. Sequences of eye movements made to successively presented targets were also impaired. Much smaller deficits were observed after MEF lesions. Our findings indicate a major, long-lasting loss in temporal ordering and processing speed for visually guided saccadic eye movement generation after FEF lesions and a significant but smaller and shorter-lasting loss after MEF lesions.
Asunto(s)
Lóbulo Frontal/fisiología , Movimientos Sacádicos/fisiología , Visión Ocular/fisiología , Campos Visuales/fisiología , Animales , Discriminación en Psicología/fisiología , Estimulación Luminosa/métodos , Primates , Tiempo de Reacción/fisiología , Factores de TiempoRESUMEN
We studied extra-receptive field contextual modulation in area V1 of awake, behaving macaque monkeys. Contextual modulation was studied using texture displays in which texture covering the receptive field (RF) was the same in all trials, but the perceptual context of this texture could vary depending on the configuration of extra-RF texture elements. We found robust contextual modulation when disparity, color, luminance, and orientation cues variously defined a textured figure centered on the RF of V1 neurons. We found contextual modulation to have a spatial extent of approximately 8 to 10 degrees diameter parafoveally. Contextual modulation correlated with perceptual experience of both binocularly rivalrous texture displays and of displays with a simple example of surface occlusion. We found contextual modulation in V1 to have a characteristic latency of 80-100 msec after stimulus onset, potentially allowing feedback from extrastriate areas to underlie to this effect.
Asunto(s)
Reconocimiento Visual de Modelos/fisiología , Corteza Visual/fisiología , Animales , Recuento de Células , Percepción de Color/fisiología , Condicionamiento Psicológico/fisiología , Potenciales Evocados Visuales/fisiología , Macaca mulatta , Masculino , Neuronas/fisiología , Factores de Tiempo , Visión Binocular/fisiología , Corteza Visual/citología , Campos Visuales/fisiologíaRESUMEN
The dominant view during the past 40 years has been that the visual system analyzes the visual scene by breaking it down into basic attributes such as color, form, motion, depth and texture. Individual dedicated neurons and specific visual areas were believed to be devoted to the analysis of each of these attributes. Current research has challenged these views by emphasizing that neurons, especially in the cortex, have multifunctional properties and therefore serve as general-purpose analyzers rather than feature detectors. Consequently, it appears that most extrastriate visual areas, rather than each being devoted to the analysis of a specific basic visual attribute, perform several different tasks and thereby engage in more advanced and complex analyses than had been realized.
Asunto(s)
Analizadores Neurales/anatomía & histología , Analizadores Neurales/fisiología , Neurociencias/tendencias , Percepción Visual/fisiología , Animales , Evolución Biológica , Historia del Siglo XIX , Historia del Siglo XX , Humanos , Modelos Neurológicos , Neurociencias/historia , Retina/anatomía & histología , Retina/fisiología , Corteza Visual/anatomía & histología , Corteza Visual/fisiología , Vías Visuales/anatomía & histología , Vías Visuales/fisiologíaRESUMEN
The primate visual system has a remarkable capability for recognizing objects irrespective of the multitude of images they form on the retinal surface by virtue of changes in size, perspective, contrast, colour and partial obstruction by other stimuli in the visual scene. There is increasing evidence that this remarkable capacity is brought about by processes that occur earlier in the visual system than had previously been thought. Here I show that after ablation of area V4 in the rhesus monkey, major deficits arise in the recognition of objects that have been transformed in size, in the degree of occlusion, and in the amount of contour information provided. The ability to detect these objects when presented individually was unaffected by these lesions.
Asunto(s)
Mapeo Encefálico , Corteza Visual/fisiología , Percepción Visual , Animales , Aprendizaje , Macaca mulatta , Movimientos Sacádicos , Percepción del Tamaño/fisiología , Corteza Visual/cirugíaRESUMEN
This study examined how variations in the visual scene affect the generation of bimodal saccadic latency distributions, the first mode of which is called the population of "express saccades". The surface media used to make stimuli visible and the composition of the background were varied to determine the conditions under which express saccades can be generated in rhesus monkeys. The results show that express saccades to singly presented targets can readily be elicited when the stimuli are made visible by virtue of either luminance contrast, color contrast or motion cues. Express saccades are rarely obtained when stimuli are made visible by virtue of only stereoscopic depth or texture cues. Express saccades can, however, be elicited using random-dot stereograms or textures when luminance or chrominance information is added to the target. When single target stimuli are presented simultaneously with a set of non-target stimuli, express saccades are for the most part prevented unless either the non-target stimuli are near threshold or their numerosity is very high, in which case they form a texture-like array. However, when the non-target stimuli are continuously present in the display, express saccades reemerge. These findings suggest that express saccades are not unique to experimental situations in which only a single stimulus appears on an otherwise homogeneous surface; they can readily be generated as long as the target stimulus is made visible by virtue of luminance, chrominance, motion or a combination of more than one surface medium and as long as the target does not appear concurrently with a salient group of other non-target stimuli.
Asunto(s)
Movimientos Sacádicos/fisiología , Percepción Visual/fisiología , Animales , Percepción de Color/fisiología , Percepción de Profundidad/fisiología , Luz , Macaca mulatta , Percepción de Movimiento/fisiología , Factores de TiempoRESUMEN
This study examined the consequences of visual system lesions on visual aftereffects produced by achromatic stimuli of various luminance contrasts and chromatic stimuli of various wavelength compositions. The effects of repeated exposure to such adapting stimuli were assessed using probes whose luminance contrast and wavelength composition were systematically varied using both detection and discrimination paradigms. Interocular tests revealed that both peripheral and central mechanisms contribute to the visual aftereffects produced by the adapting stimulus arrays used in this study. Contrary to the hypothesis according to which the midget system of the retina is the conveyor of visual afterimages, we found that blocking this system with lesions of parvocellular lateral geniculate nucleus, through which the midget cells make their way to the striate cortex in primates, did not eliminate the visual aftereffects. It appears therefore that the parasol system of the retina, which courses through the magnocellular layers of the lateral geniculate nucleus to cortex, can convey the necessary signals for the generation of visual aftereffects. Lesions of areas V4 and MT did not have significant effects on the visual aftereffects studied suggesting that the central factors that contribute to the visual aftereffects occur either already in area V1 or are conveyed to higher centers through regions other than areas V4 and MT.
Asunto(s)
Postimagen/fisiología , Cuerpos Geniculados/fisiopatología , Corteza Visual/fisiopatología , Percepción Visual/fisiología , Adaptación Ocular , Animales , Encefalopatías/fisiopatología , Percepción de Color/fisiología , Sensibilidad de Contraste , Cuerpos Geniculados/patología , Humanos , Luz , Macaca mulatta , Retina/fisiología , Corteza Visual/patología , Vías Visuales/fisiologíaRESUMEN
Visually guided saccadic eye movements to singly presented stationary targets form a bimodal distribution. After superior colliculus lesions, the so called "express saccades" that form the first mode of the distribution are no longer obtained. The aim of this study was to determine what role several other neural systems play in the generation of express and regular saccades, with the latter being those that form the second mode in the bimodal distribution. Lesions were made in the parvocellular and magnocellular portions of the lateral geniculate nucleus to disrupt either the midget system or the parasol system that originates in the retina and areas V4 and MT. The effects of the lesions were examined on the accuracy and latency of saccadic eye movements made to stationary and to moving visual targets. Following magnocellular and MT lesions deficits were observed in smooth pursuit and in the amplitude of saccades made to moving targets. However, none of the lesions produced significant changes in the bimodal distribution of saccadic latencies to stationary targets. The results suggest that express saccades and regular saccades are not selectively mediated by either the midget or the parasol systems or by areas V4 and MT. Neither are the frontal eye fields involved as had previously been shown. We suggest that the superior colliculus plays a central role in producing both express and regular saccades by virtue of highly convergent input from numerous cortical structures.
Asunto(s)
Movimientos Oculares/fisiología , Cuerpos Geniculados/fisiopatología , Movimientos Sacádicos/fisiología , Lóbulo Temporal/fisiopatología , Corteza Visual/fisiopatología , Animales , Cuerpos Geniculados/patología , Macaca mulatta , Percepción de Movimiento/fisiología , Seguimiento Ocular Uniforme/fisiología , Lóbulo Temporal/patología , Corteza Visual/patología , Vías Visuales/fisiologíaRESUMEN
Four experiments were performed to assess the effects of ON channel blockade with the glutamate analog 2-amino-4-phosphonobutyrate (APB) on brightness and contrast perception in monkeys. In Experiment 1, we demonstrate that stimuli brighter than background (incremental stimuli) appear less bright following ON channel blockade. This decrease in brightness is not enough to account for the previously observed threshold increase for detection of incremental stimuli following APB administration (Schiller et al., 1986; Dolan & Schiller, 1989). Experiment 2 examines the role of the ON and OFF channels in the interaction between local contrast and apparent brightness. The phenomenon of simultaneous contrast was examined under normal conditions and following APB administration. We find that even following ON channel blockade, the brightness of a stimulus is determined primarily by its contrast with its immediate background. This indicates that the lateral processes involved in simultaneous contrast can operate even when one channel has been compromised. In Experiment 3, we examined the role of the ON channel in detection of stimuli that appear by virtue of changes in background vs. foreground luminance. We find that the ON channel selectively conveys information pertaining not only to the temporal nature that defines the stimulus as incremental but also to the spatial features that define it as incremental. In Experiment 4, we test the hypothesis that incremental and decremental temporal luminance ramps are differentially processed by the ON and OFF channels to a higher degree than are step-luminance changes. We find that the detection of incremental ramps is no more affected than is the detection of incremental steps following APB administration.
Asunto(s)
Aminobutiratos/farmacología , Sensibilidad de Contraste/efectos de los fármacos , Interneuronas/fisiología , Retina/fisiología , Percepción Visual/efectos de los fármacos , Animales , Luz , Macaca mulatta , Umbral SensorialRESUMEN
This study examined whether signals for the generation of eye movements from the dorsomedial frontal cortex (DMFC) reach brainstem oculomotor centers either through the frontal eye fields (FEF) or through the superior colliculi (SC). The DMFC was stimulated when the monkeys studied were intact and after either one FEF or one SC was ablated. Following lesions of either the FEF or SC, the topographic order of the DMFC was largely preserved. After either lesions, stimulation of anterior DMFC sites still evoked saccades that terminated in contralateral space, and stimulation of posterior DMFC sites still evoked saccades that terminated in central space. The probability of evoking saccades decreased and the latency to evoke saccades increased as fixation neared the termination zone (a restricted region within craniotopic space) both before and after either lesion. Ablation of the SC, but not of the FEF, eliminated the saccadic inhibition to visual targets which resulted when the DMFC was stimulated in the intact animal. The findings suggest that additional channels besides those coursing through the FEF and SC are utilized by the DMFC to access the saccade generator in the brainstem.
Asunto(s)
Corteza Prefrontal/fisiología , Movimientos Sacádicos/fisiología , Colículos Superiores/fisiología , Campos Visuales/fisiología , Animales , Tronco Encefálico/anatomía & histología , Tronco Encefálico/fisiología , Estimulación Eléctrica , Fijación Ocular/fisiología , Macaca mulatta , Corteza Prefrontal/anatomía & histología , Colículos Superiores/anatomía & histología , Vías Visuales/fisiologíaRESUMEN
The effects of V4, MT, and combined V4 + MT lesions were assessed on a broad range of visual capacities that included measures of contrast sensitivity, wavelength and brightness discrimination, form vision, pattern vision, motion and flicker perception, stereopsis, and the selection of stimuli that were less prominent than those with which they appeared in stimulus arrays. The major deficit observed was a loss in the ability, after V4 lesions, to select such less prominent stimuli; this was the case irrespective of the manner in which the stimulus arrays were made visible, using either luminance, chrominance, motion, or stereoscopic depth as surface media. In addition, V4 lesions yielded mild deficits in color, brightness, and form vision whereas MT lesions yielded mild to moderate deficits in motion and flicker perception. Both lesions produced mild deficits in contrast sensitivity, shape-from-motion perception, and yielded increased reaction times on many of the tasks. The impairment resulting from combined V4 and MT lesions was not greater than the sum of the deficits of either lesion. None of the lesions produced significant deficits in stereopsis. The findings suggest that (1) area V4 is part of a neural system that is involved in extracting stimuli from the visual scene that elicit less neural activity early in the visual system than do other stimuli with which they appear and (2) several other extrastriate regions and more than just two major cortical processing streams contribute to the processing of basic visual functions in the extrastriate cortex.