Selective attention sharpens population receptive fields in human auditory cortex.

Selective attention enables the preferential processing of relevant stimulus aspects. Invasive animal studies have shown that attending a sound feature rapidly modifies neuronal tuning throughout the auditory cortex. Human neuroimaging studies have reported enhanced auditory cortical responses with selective attention.

Micropools of reliable area MT neurons explain rapid motion detection.

Many models of perceptually based decisions postulate that actions are initiated when accumulated sensory signals reach a threshold level of activity. These models have received considerable neurophysiological support from recordings of individual neurons while animals are engaged in motion discrimination tasks. These experiments have found that the activity of neurons in a particular visual area strongly associated with motion processing (MT), when pooled over hundreds of milliseconds, is sufficient to explain behavioral timing and performance.

Integration of color, orientation, and size functional domains in the ventral pathway.

Functional specialization within the extrastriate areas of the ventral pathway associated with visual form analysis is poorly understood. Studies comparing the functional selectivities of neurons within the early visual areas have found that there are more similar than different between the areas. We simultaneously imaged visually evoked activation over regions of V2 and V4 and parametrically varied three visual attributes for which selectivity exists in both areas: color, orientation, and size.

Improvement of Eye Alignment in Adult Strabismic Monkeys by Sustained IGF-1 Treatment.

Purpose: The goal of this study was to determine if continuous application of insulin-like growth factor-1 (IGF-1) could improve eye alignment of adult strabismic nonhuman primates and to assess possible mechanisms of effect.

Methods: A continuous release pellet of IGF-1 was placed on one medial rectus muscle in two adult nonhuman primates (M1, M2) rendered exotropic by the alternating monocular occlusion method during the first months of life. Eye alignment and eye movements were recorded for 3 months, after which M1 was euthanized, and the lateral and medial rectus muscles were removed for morphometric analysis of fiber size, nerve, and neuromuscular density.

Population coding in area V4 during rapid shape detections.

While previous studies have suggested that neuronal correlations are common in visual cortex over a range of timescales, the effect of correlations on rapid visually based decisions has received little attention. We trained Macaca mulatta to saccade to a peripherally presented shape embedded in dynamic noise as soon as the shape appeared. While the monkeys performed the task, we recorded from neuronal populations (5-29 cells) using a microelectrode array implanted in area V4, a visual area thought to be involved in form perception.

Vision and vigilance on the go.

Moving purposefully through the world requires the seamless coordination of a wide variety of sensory, motor, and motivation systems. Recent experiments suggest that mouse visual cortex may offer a particularly well suited forum for experimental investigation of this coordination.

Featural and temporal attention selectively enhance task-appropriate representations in human primary visual cortex.

Our perceptions are often shaped by focusing our attention towards specific features or periods of time irrespective of location. Here we explore the physiological bases of these non-spatial forms of attention by imaging brain activity while subjects perform a challenging change-detection task. The task employs a continuously varying visual stimulus that, for any moment in time, selectively activates functionally distinct subpopulations of primary visual cortex (V1) neurons.

Rapid shape detection signals in area V4.

Vision in foveate animals is an active process that requires rapid and constant decision-making. For example, when a new object appears in the visual field, we can quickly decide to inspect it by directing our eyes to the object's location. We studied the contribution of primate area V4 to these types of rapid foveation decisions.

Inattention blindness to motion in middle temporal area.

Subjects naturally form and use expectations to solve familiar tasks, but the accuracy of these expectations and the neuronal mechanisms by which these expectations enhance behavior are unclear. We trained animals (Macaca mulatta) in a challenging perceptual task in which the likelihood of a very brief pulse of motion was consistently modulated over time and space. Pulse likelihood had dramatic effects on behavior: unexpected pulses were nearly invisible to the animals.

Temporal production signals in parietal cortex.

We often perform movements and actions on the basis of internal motivations and without any explicit instructions or cues. One common example of such behaviors is our ability to initiate movements solely on the basis of an internally generated sense of the passage of time. In order to isolate the neuronal signals responsible for such timed behaviors, we devised a task that requires nonhuman primates to move their eyes consistently at regular time intervals in the absence of any external stimulus events and without an immediate expectation of reward.

Attention directed by expectations enhances receptive fields in cortical area MT.

Expectations, especially those formed on the basis of extensive training, can substantially enhance visual performance. However, it is not clear that the physiological mechanisms underlying this enhancement are identical to those examined by experiments in which attention is directed by explicit instructions rather than strong expectations. To study the changes in visual representations associated with strong expectations, we trained animals to detect a brief motion pulse that was embedded in noise.

Attentional modulation of visual responses by flexible input gain.

Although it is clear that sensory responses in the cortex can be strongly modulated by stimuli outside of classical receptive fields as well as by extraretinal signals such as attention and anticipation, the exact rules governing the neuronal integration of sensory and behavioral signals remain unclear. For example, most experiments studying sensory interactions have not explored attention, while most studies of attention have relied on the responses to relatively limited sets of stimuli. However, a recent study of V4 responses, in which location, orientation, and spatial attention were systematically varied, suggests that attention can both facilitate and suppress specific sensory inputs to a neuron according to behavioral relevance.

Temporal precision of neuronal information in a rapid perceptual judgment.

In many situations, such as pedestrians crossing a busy street or prey evading predators, rapid decisions based on limited perceptual information are critical for survival. The brevity of these perceptual judgments constrains how neuronal signals are integrated or pooled over time because the underlying sequence of processes, from sensation to perceptual evaluation to motor planning and execution, all occur within several hundred milliseconds. Because most previous physiological studies of these processes have relied on tasks requiring considerably longer temporal integration, the neuronal basis of such rapid decisions remains largely unexplored.

Spatial summation can explain the attentional modulation of neuronal responses to multiple stimuli in area V4.

Although many studies have shown that the activity of individual neurons in a variety of visual areas is modulated by attention, a fundamental question remains unresolved: can attention alter the visual representations of individual neurons? One set of studies, primarily relying on the attentional modulations observed when a single stimulus is presented within the receptive field of a neuron, suggests that neuronal selectivities, such as orientation or direction tuning, are not fundamentally altered by attention (Salinas and Abbott, 1997; McAdams and Maunsell, 1999; Treue and Martinez Trujillo, 1999). Another set of studies, relying on modulations observed when multiple stimuli are presented within a receptive field, suggests that attention can alter the weighting of sensory inputs (Moran and Desimone, 1985; Luck et al., 1997; Reynolds et al.

Spatial attention does not strongly modulate neuronal responses in early human visual cortex.

Attention can dramatically enhance behavioral performance based on a visual stimulus, but the degree to which attention modulates activity in early visual cortex is unclear. Whereas single-unit studies of spatial attention in monkeys have repeatedly revealed relatively modest attentional modulations in V1, human functional magnetic resonance imaging studies demonstrate a large attentional enhancement of the blood oxygen level-dependent (BOLD) signal in V1. To explore this discrepancy, we used intracranial electrodes to directly measure the effect of spatial attention on the responses of neurons near the human occipital pole.

Receptive fields in human visual cortex mapped with surface electrodes.

Most of our understanding of the functional organization of human visual cortex comes from lesion and functional imaging studies and by extrapolation from results obtained by neuroanatomical and neurophysiological studies in nonhuman primates. Although some single-unit and field potential recordings have been made in human visual cortex, none has provided quantitative characterization of spatial receptive fields (RFs) of individual sites. Here we use subdural electrodes implanted for clinical purposes to quantitatively measure response properties in different regions of human visual cortex.

Strategies optimize the detection of motion transients.

Strategies are implicitly formed when a task is consistent and can be used to improve performance. To investigate how strategies can alter perceptual performance, I trained animals in a reaction time (RT) detection task in which the probability of a fixed duration motion pulse appearing varied over time in a consistent manner. Consistent with previous studies suggesting the implicit representation of task timing, I found that RTs were inversely related to the probability of the pulse appearing and decreased with training.

Learning in mammalian sensory cortex.

The improvement of perceptual capabilities with training can offer important insight into the physiological basis of learning in the cerebral cortex. The rapid time course and ease with which some perceptual capabilities can improve suggest that learning is an integral part of normal perception. Electrophysiological and neuroimaging studies of sensory systems in the cortex suggest that the changes underlying perceptual learning can occur in a variety of areas and are likely to involve multiple mechanisms.

Attentional modulation in visual cortex depends on task timing.

Paying attention to a stimulus selectively increases the ability to process it. For example, when subjects attend to a specific region of a visual scene, their sensitivity to changes at that location increases. A large number of studies describe the behavioural consequences and neurophysiological correlates of attending to spatial locations.

Physiological correlates of perceptual learning in monkey V1 and V2.

Performance in visual discrimination tasks improves with practice. Although the psychophysical parameters of these improvements have suggested the involvement of early areas in visual cortex, there has been little direct study of the physiological correlates of such perceptual learning at the level of individual neurons. To examine how neuronal response properties in the early visual system may change with practice, we trained monkeys for more than 6 mo in an orientation discrimination task in which behaviorally relevant stimuli were restricted to a particular retinal location and oriented around a specific orientation.