CiteULike: klouie Maunsell

Shape selectivity in primate lateral intraparietal cortex.

AB Sereno — 2008-06-25T17:42:18-00:00

Nature, Vol. 395, No. 6701. (1 October 1998), pp. 500-503.

The extrastriate visual cortex can be divided into functionally distinct temporal and parietal regions, which have been implicated in feature-related ('what') and spatial ('where') vision, respectively. Neuropsychological studies of patients with damage to either the temporal or the parietal regions provide support for this functional distinction. Given the prevailing modular theoretical framework and the fact that prefrontal cortex receives inputs from both temporal and parietal streams, recent studies have focused on the role of prefrontal cortex in understanding where and how information about object identity is integrated with (or remains segregated from) information about object location. Here we show that many neurons in primate posterior parietal cortex (the 'where' pathway) show sensory shape selectivities to simple, two-dimensional geometric shapes while the animal performs a simple fixation task. In a delayed match-to-sample paradigm, many neuronal units also show significant differences in delay-period activity, and these differences depend on the shape of the sample. These results indicate that units in posterior parietal cortex contribute to attending to and remembering shape features in a way that is independent of eye movements, reaching, or object manipulation. These units show shape selectivity equivalent to any shown in the ventral pathway.

Electrical microstimulation thresholds for behavioral detection and saccades in monkey frontal eye fields

Dona Murphey — 2008-05-27T15:49:43-00:00

Proceedings of the National Academy of Sciences (13 May 2008), 0710820105.

The frontal eye field (FEF) is involved in the transformation of visual signals into saccadic eye movements. Although it is often considered an oculomotor structure, several lines of evidence suggest that the FEF also contributes to visual perception and attention. To better understand the range of behaviors to which the FEF can contribute, we tested whether monkeys could detect activation of their FEF by electrical microstimulation with currents below those that cause eye movements. We found that stimulation of FEF neurons could almost always be detected at levels below those needed to generate saccades and that the electrical current needed for detection was highly correlated with that needed to generate a saccade. This relationship between detection and saccade thresholds can be explained if FEF neurons represent preparation to make particular saccades and subjects can be aware of such preparations without acting on them when the representation is not strong. 10.1073/pnas.0710820105

Feature-based attention in visual cortex.

JH Maunsell — 2007-03-01T17:52:53-00:00

Trends Neurosci, Vol. 29, No. 6. (June 2006), pp. 317-322.

Although most studies of visual attention have examined the effects of shifting attention between different locations in the visual field, attention can also be directed to particular visual features, such as a color, orientation or a direction of motion. Single-unit studies have shown that attention to a feature modulates neuronal signals in a range of areas in monkey visual cortex. The location-independent property of feature-based attention makes it particularly well suited to modify selectively the neural representations of stimuli or parts within complex visual scenes that match the currently attended feature. This review is part of the TINS special issue on The Neural Substrates of Cognition.

Spatial attention and the latency of neuronal responses in macaque area V4.

J Lee — 2007-10-03T03:24:26-00:00

J Neurosci, Vol. 27, No. 36. (5 September 2007), pp. 9632-9637.

The effects of attention on neuronal responses in visual cortex have been likened to a change in stimulus contrast. Attention and stimulus contrast both modulate the magnitude of neuronal responses. However, changes in stimulus contrast also affect the latency of visual responses. Although many neurophysiological studies have examined how attention affects the strength of neuronal responses, few have considered whether attention affects neuronal latencies. To compare directly the effects of stimulus contrast and attention, we recorded responses from individual neurons in area V4 of macaque monkeys while they performed a task that independently controlled spatial attention and stimulus contrast. As expected, changes in stimulus contrast affected both the magnitude and latency of neuronal responses. Although attention had the expected effects on the magnitudes of neuronal responses, we did not detect statistically reliable changes in neuronal latency. A direct comparison of the effects of contrast and attention revealed a reliable difference. When a shift in spatial attention decreased response magnitude, response latency increased much less than when the same magnitude change was caused by reducing stimulus contrast. Thus, attention is distinct from contrast in the way it affects the relationship between neuronal response magnitude and latency.

Behavioral detection of electrical microstimulation in different cortical visual areas.

DK Murphey — 2007-05-22T14:23:42-00:00

Curr Biol, Vol. 17, No. 10. (15 May 2007), pp. 862-867.

The extent to which areas in the visual cerebral cortex differ in their ability to support perceptions has been the subject of considerable speculation. Experiments examining the activity of individual neurons have suggested that activity in later stages of the visual cortex is more closely linked to perception than that in earlier stages [1-9]. In contrast, results from functional imaging, transcranial magnetic stimulation, and lesion studies have been interpreted as showing that earlier stages are more closely coupled to perception [10-15]. We examined whether neuronal activity in early and later stages differs in its ability to support detectable signals by measuring behavioral thresholds for detecting electrical microstimulation in different cortical areas in two monkeys. By training the animals to perform a two-alternative temporal forced-choice task, we obtained criterion-free thresholds from five visual areas-V1, V2, V3A, MT, and the inferotemporal cortex. Every site tested yielded a reliable threshold. Thresholds varied little within and between visual areas, rising gradually from early to later stages. We similarly found no systematic differences in the slopes of the psychometric detection functions from different areas. These results suggest that neuronal signals of similar magnitude evoked in any part of visual cortex can generate percepts.

Neuronal representations of cognitive state: reward or attention?

John Maunsell — 2005-10-05T14:41:04-00:00

Trends in Cognitive Sciences, Vol. 8, No. 6. (1 June 2004), pp. 261-265.

The effects of spatial or featural attention on the activity of neurons have been studied in many experiments that have used a variety of neurophysiological approaches. Other experiments have examined how expectations about reward are represented in neuronal activity in various brain regions. Although attention and reward are distinct concepts, I argue here that many neurophysiological experiments on attention and reward do not permit a clean dissociation between the two. This problem arises in part because reward contingencies are the only parameter manipulated in any of these experiments. I describe how attention and reward expectations have been confounded, giving rise to uncertainty about how signals related to attention and reward are distributed in the brain.

Using neuronal latency to determine sensory-motor processing pathways in reaction time tasks.

JJ DiCarlo — 2006-05-22T22:14:52-00:00

J Neurophysiol, Vol. 93, No. 5. (May 2005), pp. 2974-2986.

We describe a new technique that uses the timing of neuronal and behavioral responses to explore the contributions of individual neurons to specific behaviors. The approach uses both the mean neuronal latency and the trial-by-trial covariance between neuronal latency and behavioral response. Reliable measurements of these values were obtained from single-unit recordings made from anterior inferotemporal (AIT) cortex and the frontal eye fields (FEF) in monkeys while they performed a choice reaction time task. These neurophysiological data show that the responses of AIT neurons and some FEF neurons have little covariance with behavioral response, consistent with a largely "sensory" response. The responses of another group of FEF neurons with longer mean latency covary tightly with behavioral response, consistent with a largely "motor" response. A very small fraction of FEF neurons had responses consistent with an intermediate position in the sensory-motor pathway. These results suggest that this technique is a valuable tool for exploring the functional organization of neuronal circuits that underlie specific behaviors.

Neuronal correlates of inferred motion in primate posterior parietal cortex.

JA Assad — 2005-02-08T21:50:34-00:00

Nature, Vol. 373, No. 6514. (9 February 1995), pp. 518-521.

For many types of behaviours, it is necessary to monitor the position or movement of objects that are temporarily occluded. The primate posterior parietal cortex contains neurons that are active during visual guidance tasks: in some cases, even if the visual target disappears transiently. It has been proposed that activity of this sort could be related to current or planned eye movements, but it might also provide a more generalized abstract representation of the spatial disposition of targets, even when they are not visible. We have recorded from monkey posterior parietal cortex while the animal viewed a visual stimulus that disappeared, and then, depending on experimental context, could be inferred to be either moving or stationary. During this temporary absence of the stimulus, about half of the neurons were found to be significantly more active on those trials in which the stimulus could be presumed to be moving rather than stationary. The activity was thus present in the absence of either sensory input or motor output, suggesting that it may indeed constitute a generalized representation of target motion.