ABSTRACT
The duration of the visual search by human participants for visual features is independent of the number of targets being viewed. In contrast, search for targets formed by conjunction of features is characterized by reaction times that increase as a linear function of the number of items viewed, suggesting that the target detection requires scrutiny of the search array by focal attention. Macaque (Macaca mulatta) and human performance on feature and conjunction search tasks was compared by using color or motion, or by conjunctions of color and motion. Like human participants, monkeys exhibited a dichotomy between feature and conjunction search performance. This finding suggests that humans and macaques engage similar brain mechanisms for representation of feature and conjunction targets. This behavioral paradigm can thus be used in neurophysiological experiments directed at the mechanisms of feature integration and target selection.
Subject(s)
Color Perception , Motion Perception , Psychomotor Performance , Signal Detection, Psychological , Adult , Animals , Female , Humans , Macaca mulatta , Male , Models, Psychological , Reaction Time , RewardABSTRACT
The stream of information that enters a sensory system is a product of the ecological niche of an organism and the way in which the information is sampled. The most salient characteristic of this sensory stream is the rich temporal structure that is caused by changes in the environment and self motion of sensors (for example, rapid eye or whisker movements). In recent years, substantial progress has been made in understanding how such rapidly varying stimuli are represented in the responses of sensory neurons of a large variety of sensory systems. The crucial observation that has emerged from these studies is that individual action potentials convey substantial amounts of information, which permits the discrimination of rapidly varying stimuli with high temporal precision.
Subject(s)
Brain/physiology , Mental Processes/physiology , Nerve Net/physiology , Neurons, Afferent/physiology , Action Potentials/physiology , Animals , Humans , Time FactorsABSTRACT
Although motion-sensitive neurons in macaque middle temporal (MT) area are conventionally characterized using stimuli whose velocity remains constant for 1-3 s, many ecologically relevant stimuli change on a shorter time scale (30-300 ms). We compared neuronal responses to conventional (constant-velocity) and time-varying stimuli in alert primates. The responses to both stimulus ensembles were well described as rate-modulated Poisson processes but with very high precision (approximately 3 ms) modulation functions underlying the time-varying responses. Information-theoretic analysis revealed that the responses encoded only approximately 1 bit/s about constant-velocity stimuli but up to 29 bits/s about the time-varying stimuli. Analysis of local field potentials revealed that part of the residual response variability arose from "noise" sources extrinsic to the neuron. Our results demonstrate that extrastriate neurons in alert primates can encode the fine temporal structure of visual stimuli.
Subject(s)
Haplorhini/physiology , Motion Perception/physiology , Neurons, Afferent/physiology , Time Perception/physiology , Visual Cortex/physiology , Action Potentials/physiology , Algorithms , Analysis of Variance , Animals , Attention/physiology , Discrimination Learning/physiology , Information Theory , Nonlinear Dynamics , Photic Stimulation , Time FactorsABSTRACT
Successful recognition and manipulation of objects in one's visual environment is critically dependent upon the ability to recover three-dimensional (3D) surface geometry from two-dimensional (2D) retinal images. The relative motion of image features, caused by relative displacement of object and observer, has characteristic properties that betray components of the 3D source geometry (distance, tilt, slant and curvature) and is among the most valuable sources of information used for 3D surface recovery by the primate visual system. We have considered the behavior of motion-sensitive neurons in primate visual cortex and found that their properties closely resemble those of differential motion operators that can be used to formally characterize the 3D shape of a smooth moving surface. Our analysis has led us to identify a set of three orders of filters for differential motion detection. These filters behave in a manner that is strikingly similar to the spatial and velocity tuning profiles of a sub-population of neurons--those possessing antagonistic motion surrounds--in the middle temporal visual area (MT). On the basis of this analysis, we suggest that MT neurons subserve 3D surface recovery from relative motion cues.
