State dependent activity in monkey visual cortex. I. Single cell activity in V1 and V4 on visual tasks

The effects of V4 and middle temporal (MT) area lesions on visual performance in the rhesus monkey

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.

Attentional control of early perceptual learning

The performance of adult humans in simple visual tasks improves dramatically with practice. This improvement is highly specific to basic attributes of the trained stimulus, suggesting that the underlying changes occur at low-level processing stages in the brain, where different orientations and spatial frequencies are handled by separate channels. We asked whether these practice effects are determined solely by activity in stimulus-driven mechanisms or whether high-level attentional mechanisms, which are linked to the perceptual task, might control the learning process. We found that practicing one task did not improve performance in an alternative task, even though both tasks used exactly the same visual stimuli but depended on different stimulus attributes (either orientation of local elements or global shape). Moreover, even when the experiment was designed so that the same responses were associated with the same stimuli (although subjects were instructed to attend to the attribute underlying one task), learning did not transfer from one task to the other. These results suggest that specific high-level attentional mechanisms, controlling changes at early visual processing levels, are essential in perceptual learning.

Focal visual attention produces illusory temporal order and motion sensation

Spatial attention was studied using a new visual illusion of motion: a line, which was presented physically at once, was perceived to be drawn from one side when attention had been captured to that side of the line by a preceding visual cue stimulus. By comparing with a temporal order task, we showed that the line-motion illusion was produced by acceleration of visual information processing at the locus of attention. The results suggest that the facilitatory effect of attention is exerted at relatively early stages of visual information processing where visual signals are to be fed into the motion detecting mechanism.

Functional comparison of neuronal properties in the primate posterior hippocampus and parahippocampus (area TF/TH) during different behavioural paradigms involving memory and selective attention

Monkeys were trained on a delayed match-to-sample (DMS) task. In addition a standardized behavioural trial was performed which involved an experimenter approaching the animal in certain sequence and presenting edible or other objects ('raisin trial'). Neuronal activity of 617 units was recorded in the posterior parahippocampus (PH) and in the posterior hippocampus (H). In many cases, we compared the activity of the same neuron in different tasks. 32.7% of the 455 PH neurons and 28.5% of the 130 H cells responded during the presentation of the visual stimuli in the DMS task. These responses were only mildly influenced by the physical dimensions of the visual stimulus, but often depended on the context in which the stimuli were presented. There was no differential response to the second stimulus that clearly depended on the nature of the first stimulus. 6.2% of the PH units, but none in H, responded in relation to the reward. 4.4% of the PH neurons, but none in H, showed a mild response during the interstimulus interval. 38.1% of 215 PH neurons and 37.8% of 45 H cells responded during one or more phases of the raisin trial. These responses were not related to the physical dimensions of the sensory stimuli. 210 PH and 41 H units were investigated during the DMS task as well as during the raisin trial. 18.1% (PH) and 12.2% (H) of the units responded during the DMS task, but not during the raisin trial; 17.1% (PH) and 36.6% (H) responded vice versa. A response in both trials was found in 17.1% of the PH neurons, but in none of the H cells. There were also other PH unit types showing responses during different aspects of the DMS task and even in other control paradigms, while no such overlap was encountered in H. Our results suggest a function of H and PH in the evaluation of the behavioural significance of sensory information. It may be this aspect which leads to anterograde memory disturbances after lesion of these areas. Since representation of neuronal information was found to be more specific in H, a possible function as an 'evaluation index' is discussed.

The effects of V4 lesions on the visual abilities of macaques: shape discrimination

Monkeys with bilateral ablation of cortical visual area V4 were compared with unoperated controls for their ability to relearn postoperatively a series of preoperatively acquired two-choice visual discrimination problems. The animals with V4 lesions were impaired on relearning to discriminate between different shapes, and discrimination between identical shapes presented at different orientations was also impaired. Some of the deficits were consistent with disrupting the input to inferotemporal cortex, but discrimination of a subset of the stimuli is known to be unaffected by inferotemporal cortex lesions and could not be explained in the same way. To clarify the nature of the deficit, and to test the hypothesis that shapes differing in orientation are analyzed in the occipitoparietal processing pathway, animals with V4 lesions were also compared to normals on their ability to acquire a version of the landmark task. The V4 animals performed as well as the control animals on this task. The results suggest that V4 is important for the shape discrimination abilities that survive inferotemporal cortex lesions. The role of V4 in shape analysis is discussed in the light of recent evidence that V4 neurones are modulated by visual attention.

Extraretinal representations in area V4 in the macaque monkey

Several neurophysiological studies have shown that the visual cerebral cortex of macaque monkeys performing delayed match-to-sample tasks contains individual neurons whose levels of activity depend on the sample the animal is required to remember. Haenny et al. (1988) reported that the activity of neurons in area V4 of monkeys performing an orientation matching task depends on the orientation for which the animal is searching. It was proposed that these neurons contribute to a representation of the orientation being sought. We have further characterized these neurons by recording visual responses from individual neurons during multiple behavioral tasks. Animals were trained to perform an orientation match-to-sample task using either a visual or a tactile orientation sample. In a set of 89 neurons examined using both types of sample, 25% showed statistically significant effects of sample orientation regardless of whether the sample was visual or tactile. Most of these preferred the same sample orientation in both conditions. These results allow us to specify the nature of the information signaled by these neurons more precisely than has previously been possible. For 193 units tested using one of the matching tasks, responses were also recorded while the animal performed a simple fixation task. In this task the animal was not required to attend to the visual stimuli that were presented. A few neurons that were responsive during the matching task were silent during fixation, but a comparable number was much more responsive during fixation. Across the whole population there was no systematic change in either responsivity or selectivity for orientation under the two conditions.

A neural mechanism for working and recognition memory in inferior temporal cortex

Inferior temporal (IT) cortex is critical for visual memory, but it is not known how IT neurons retain memories while new information is streaming into the visual system. Single neurons were therefore recorded from IT cortex of two rhesus monkeys performing tasks that required them to hold items in memory while concurrently viewing other items. The neuronal response to an incoming visual stimulus was attenuated if it matched a stimulus actively held in working memory, even when several other stimuli intervened. The neuronal response to novel stimuli declined as the stimuli became familiar to the animal. IT neurons appear to function as adaptive mnemonic "filters" that preferentially pass information about new, unexpected, or not recently seen stimuli.

Habituation-like decrease in the responses of neurons in inferior temporal cortex of the macaque

In both anesthetized and behaving macaques, we examined the responses of neurons in the inferior temporal cortex (IT) to repeated presentation of a visual stimulus. In anesthetized animals, the responsiveness of IT neurons decreased with repeated stimulus presentation at interstimulus intervals (ISIs) of 2-12 s but not at 20 s. Responsiveness recovered after a 5-min period of no stimulus presentation. The response decrement was similar in anesthetized and awake animals at a 2-s ISI, but at a 6-s ISI, response decrement in the awake animal was much less.

Unit activity in the hippocampus and the parahippocampal temporobasal association cortex related to memory and complex behaviour in the awake monkey

Monkeys (Macaca fascicularis) were trained on a delayed match-to-sample (DMS) task using delays of upto 20 s. Unit activity was recorded from the hippocampus and the temporo-basal association cortex in the lateral parahippocampal region (partly corresponding to TF and TH) during the DMS task, as well as during a visual object discrimination task and some behavioural situations involving the experimenter. Units were encountered that gave visual responses which were sometimes context-dependent. Changes in discharge rate during the delay period of the DMS task were very rare and when present, very weak. On the other hand, many neurones, including some of those which were unresponsive during the DMS task fired vigorously (or were inhibited) during situations which involved attention, expectation or food consumption. For example, the neurones' firing rate was altered when the cage door was opened or closed, the experimenter entered or left the room or showed the monkey a piece of food before giving it to him. A variety of such responses in complex behavioural situations were seen, sometimes even in neurones which did not respond in the DMS task. Activity changes in neurons of the temporo-basal cortex thus appear to be related to the internal state associated with a stimulus and even some of the responses obtained in the DMS task can be interpreted as being related to changes in the behavioural state rather than to the mnemonic elements of the task.

The role of the primate extrastriate area V4 in vision

Area V4 is a part of the primate visual cortex. Its role in vision has been extensively debated. Inferences about the functions of this area have now been made by examination of a broad range of visual capacities after ablation of V4 in rhesus monkeys. The results obtained suggest that this area is involved in more complex aspects of visual information processing than had previously been suggested. Monkeys had particularly severe deficits in situations where the task was to select target stimuli that had a lower contrast, smaller size, or slower rate of motion than the array of comparison stimuli from which they had to be discriminated. Extensive training on each specific task resulted in improved performance. However, after V4 ablation, the monkeys could not generalize the specific task to new stimulus configurations and to new spatial locations.

Visual 'cognitive' evoked potentials in the behaving monkey

Using a visual 'oddball' paradigm we studied ERPs in monkeys trained in a 'go' 'no-go' discrimination task. The stimuli were 2.5 cpd sinusoidal gratings differing only in orientation (0 degrees or 25 degrees). Monkeys released a lever during 1 of 2 response windows (RW), 480-1762 or 740-1672 msec, following target stimulus onset. Target stimulus presentation probabilities were 1.0, 0.5 and 0.3. The primary evoked potentials recorded to either the target or non-target stimulus were similar in all monkeys. P3 signals progressively emerged in the monkeys only to the target stimulus. P3 recorded at Cz, P3, and P4 had similar mean latencies and amplitudes. Eye movements showed no relationship to P3 potentials. Neither the primary visual potentials nor P3 changed significantly as a function of RW. P3 amplitude was inversely related to target probability. When the target stimulus was presented 100% of the time (P = 1.0) P3 disappeared over 4-5 blocks of trials, while the primary evoked potentials remained consistent.

Color in visual search

Colored targets pop out of displays under conditions in which the standard red-green, yellow-blue and black-white mechanisms cannot directly mediate detection. Experimental evidence suggests that observers possess chromatic detection mechanisms tuned to intermediate hues such as orange as well as to hues characterizing the standard color-opponent mechanisms and that these mechanisms, as a group, form a fine-grained representation of hue within the central visual field. Spatially-parallel search is mediated by a single such mechanism that is spectrally sensitive to the target chromaticity but insensitive to the distractor chromaticities; different mechanisms are used to detect a single target in a way that depends on distractor chromaticities.

Attentional modulation of neural processing of shape, color, and velocity in humans

Positron emission tomography (PET) was used to measure changes in regional cerebral blood flow of normal subjects, while they were discriminating different attributes (shape, color, and velocity) of the same set of visual stimuli. Psychophysical evidence indicated that the sensitivity for discriminating subtle stimulus changes was higher when subjects focused attention on one attribute than when they divided attention among several attributes. Correspondingly, attention enhanced the activity of different regions of extrastriate visual cortex that appear to be specialized for processing information related to the selected attribute.

Attention and eye movement related activation of neurons in the dorsal prelunate gyrus (area DP)

Neurons in area DP, the dorsomedial portion of the prelunate gyrus of awake monkeys (Macaca mulatta and Macaca sylvana), responded only little if at all to stationary or moving light stimuli. Circumscribed receptive fields could not be determined in the majority of cells. About 25% of the units became active at a certain gaze position, mostly ipsilateral to the recording site, and with a latency of 70-150 ms after the eye had attained this position with a saccade. About 70% of neurons were activated vigorously when the monkey looked attentively at an object, such as a face, a glove, a hand or simply towards the opening door, and explored it visually. These stimuli elicit attention as well as emotional responses. Photographed objects or faces flashed on a screen produced only little if any response. Our observations, therefore, suggest that the dorsomedial part of the prelunate gyrus may represent activities related to behavioral aspects of vision rather than to features of the visual image itself.

The physiology of attention: participation of cat striate cortex in behavioral choice

In the awake, behaving cat, we have compared response of striate cortex neurons to informative and to noninformative stimuli. A cat pressed a pedal in response to a flashed pattern repeated every 10 s, receiving a reward if the press was within a 0.5-1.5-s post-stimulus window. There were three trial types: 50% of the trials had only the initial informative flash, 25% had an additional physically identical, but unrewarded, flash 500 ms after the first, and 25% had an unrewarded flash 3-5 s after the informative flash. Cats learned to respond only to the rewarded flashes. Neurons were divided into two categories: 27 neurons defined as "primary" showed an early burst of firing 30-70 ms after stimulus onset, and 17 did not. The distinction was arbitrary, since all cells were exposed to the same stimulus. For stimuli preceding a pedal press, stimulus-synchronized histograms of primary neurons had a smaller early burst and more firing before and after it. Response-synchronized histograms showed an abrupt decrease in firing shortly before the pedal press. The effects were stronger for primary cells in the stimulus-synchronized data and stronger for non-primary cells in the response-synchronized data.

Timing and distribution of flash-evoked activity in the lateral geniculate nucleus of the alert monkey

Simultaneous recording of activity from multiple cortical laminae in alert monkeys, using multichannel electrodes, has been used to identify the intracranial generators of surface-recorded, visually evoked potentials (VEP) to stroboscopic flash. Beyond their clinical implications, these results offer an unique view of the timing and sequence of cortical visual processing in the alert monkey, including the somewhat surprising findings of an extremely short-latency response in lamina IVA, a contra- over ipsilateral latency advantage throughout lamina IV, and the lack of a consistent flash-evoked response in the major cortical recipient of the magnocellular system, lamina IVCa. The present study used similar techniques to examine flash-evoked activity in LGN and in optic tract, both to elucidate the role of the subcortical pathways in establishing this pattern, and to provide a parallel, detailed view of the timing of visual activity in LGN and optic tract in the alert monkey. Flash-evoked responses are robust in both parvo- and magnocellular laminae, but these responses differ along several dimensions: (1) parvocellular multiunit activity (MUA) is 1/4 to 1/2 the amplitude of magnocellular MUA; (2) oscillatory activity is higher in frequency and shorter in duration in parvo- than in magnocellular responses; (3) inhibitory processes appear less prominent and diverse in parvo- than in magnocellular activity; (4) mean onset latencies of MUA are longer in parvo- than in magnocellular laminae, but there is extensive overlap in these distributions. Latencies encountered in ipsilateral lamina 3, and at laminar borders dorsal to 3, group more clearly with those of the magnocellular laminae than with those of the other parvocellular laminae. As a result, in the parvocellular division as a whole, the average latency to ipsilateral stimulation is shorter than that to contralateral stimulation. The optic tract exhibits a dorsal-to-ventral progression of onset latency and oscillation frequency consistent with a dorsal/ventral segregation of the inputs to parvo- and magnocellular layers. Comparison of optic tract and LGN data reveals that while many LGN response characteristics are initiated in the retina, significant modification of retinal output occurs at LGN. The techniques used here permit a particularly sensitive and reliable assessment of the timing and distribution of visual responses in the optic tract and LGN of alert monkeys. Our data support the view that in the alert monkey, the surface-VEP to passive, binocular flash primarily reflects activation of parvocellular thalamorecipient laminae of Area 17.(ABSTRACT TRUNCATED AT 400 WORDS)