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Matthews, J. (1994). Neurobiology of Visual Perception: Neuronal Correlates of a Perceptual Decision. W. T. Newsome; K. H. Britten; J. A. Movshon. Nature. 341, 1989. Pp. 52-54.. Psychoanal Q., 63:393-394.
Psychoanalytic Electronic Publishing: Neurobiology of Visual Perception: Neuronal Correlates of a Perceptual Decision. W. T. Newsome; K. H. Britten; J. A. Movshon. Nature. 341, 1989. Pp. 52-54.
Cortical Microstimulation Influences Perceptual Judgments of Motion Direction. C. D. Salzman; K. H. Britten; W. T. Newsome. Nature. 346, 1990. Pp. 174-177.
To understand the basis of this feature-selective response, numerous electrophysiological studies have investigated the stimulus selectivity of single neurons and neuron groups in different cortical areas. Many neurons in the primary visual cortex are optimally responsive to simple stimuli such as light bars with a particular orientation, while single neurons in area V4 (color) respond maximally to stimuli of specific wavelength. Single neurons in the middle temporal area (MT, V5) of the extrastriate cortex are selectively responsive to motion in a preferred direction, and conversely are nonresponsive to motion in the opposite direction. As is the case for other sensory areas, neurons in V5 are arranged in columns such that there exists a series of cell columns selectively responsive to each motion direction for each region of the visual field.
Newsome and colleagues have begun to explore the relationship between neuronal activity and perception. The first of these papers correlates single cell responses to moving stimuli with the perceptual judgment of alert behaving monkeys. Macaque monkeys were trained to search for coherent movement of a subset of dots within a visual display of dots moving in random directions, and to "report" the
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direction of movement by a directed eye movement following the cessation of the visual stimulus. Single cell recordings were obtained from cells in area V5 while the alert monkey performed this discrimination task. The visual stimulus was matched to the field position and motion selectivity of each neuron examined. The strength of the motion stimulus was varied by altering the proportion of dots moving coherently in the defined direction, allowing determination of a probability function for correct responses as a function of percent dot coherence (the psychometric function). A comparable function was derived for the single neuron response by determining the threshold level of coherent dot movement at which the neuron selectively responded. This neurometric function for a single cell typically paralleled the psychometric function and had a similar response threshold.
In a second study, Salzman, et al., investigated the effect of simultaneous cortical microstimulation on the performance of the monkeys on the same motion discrimination task. Once again the visual stimulus was matched to the selective sensitivity of the cortical neurons under study. Bursts of 10 microamp pulses were applied coincident with the visual stimulus (sufficient to activate neurons locally within an area of approximately 85 microns). The psychometric function for motion discrimination was significantly shifted when bursts of microstimuli were applied to the area of cortex identified as maximally sensitive to the visual stimulus under study. In other words, the microstimulation lowered the perceptual threshold, biasing the monkey's judgment in favor of detecting the motion.
These studies demonstrate that activity in small clusters of neurons can affect perceptual discrimination at the preconscious or conscious level, i.e., at the level of behavior.
The above findings have the important implication that the visual image as subjectively perceived is not localizable to any single site. Visual perception is a construction within the brain rather than a photographic replica of the external world. Since the various perceptual features are analyzed in anatomically distinct areas, there must be a mechanism to integrate these features into a coherent perception, commonly referred to as the "binding problem." A leading hypothesis builds on the observation that visually stimulated neuronal activity tends to occur with a characteristic spike frequency of around 40 cycles per second (40 Hz). Synchronized oscillation may "bind" the components of the visual image and form the basis of perception, as described below.
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Matthews, J. (1994). Neurobiology of Visual Perception. Psychoanal. Q., 63:393-394