Integration across Space

The early processing of motion is local; receptive fields of motion-sensitive neurons in area V1 are small (approximately 0.3° at an eccentricity of 1°). Receptive fields at comparable eccentricities in area MT are at least a factor of 3 larger. Although the pooling of motion signals across space reduces the accuracy of local motion estimates, it is useful for measuring large field motion in the presence of noise. Bill Newsome and colleagues demonstrated the ability of neurons in area MT to integrate local motion signals. Their stimulus was a large patch of dots, which were divided into two groups, signal and noise. Each noise dot was plotted at a new random position in each frame, but the signal dots all moved in a consistent direction. Awake behaving monkeys were trained to respond to the direction of motion of the signal dots and the response ofindividual neurons was recorded. The solid circles in Fig. 6A plot the proportion of correct trials as a function of the percentage of correlated dots. The solid curve through these points is the psychometric function. This function is usually summarized by the coherence threshold, which is the smallest fraction of signal dots that supports criterion performance in the animal. These researchers also showed that a neuron with a preferred direction that matched the signal had a response that increased with the proportion of coherent dots in the display. Furthermore, from the distribution of the neuron's responses to stimuli of various coherence levels, they were able to construct a neurometric function, the equivalent of a psychometric function for a neuron. The open circles and dashed line in Fig. 6A represent a neurometric function, which plots the probability that the neuron's response to stimulus motion in its preferred direction is larger than its response to stimulus motion in the opposite direction as a function of the stimulus coherence level. Thus, a neurometric coherence threshold can be measured in a manner analogous to the behavioral threshold. The resulting neurometric thresholds for the most sensitive MT cells are similar to the psychometric thresholds of awake behaving animals, and both are significantly lower than those predicted from measurements of local V1 units. This makes a good case for the role of MT in the integration of local motion signals.

Two further studies strengthen the role of MT neurons in the integration of this large field stimulus. Punctate lesions in area MT caused severe deficits in the ability of the animal to do the task when the stimulus was presented in the visual field location corresponding to the lesion site. The corresponding location in the (intact) contralateral hemifield served as a control; the monkey's response was unimpaired when the stimulus was presented at this location. Furthermore, electrical microstimulation of a neuron increased the probability that the monkey's perceptual response would be in the stimulated cell's preferred direction. Figure 6B plots psychometric functions with and without stimulation (dashed and solid lines, respectively). Note that the y axis is the proportion

Figure 6 (A) Behavioral and neural responses to the same motion stimuli. The proportion of correct responses is plotted as a function of varying motion coherence as illustrated below the horizontal axis. The open and solid symbols represent psychophsyical and phsyiological data, respectively. (B) Psychometric functions measured with and without electrical stimulation of the neuron being recorded (solid and filled symbols, respectively). The preferred direction of the cell was upward, so the vertical axis plots the proportion of upward responses. In the absence of stimulation, an uncorrelated display (0% coherence) evoked 50% upward responses (chance performance). In the presence of stimulation, the function was shifted leftward so that an uncorrelated display evoked a significant proportion of upward responses.

Figure 6 (A) Behavioral and neural responses to the same motion stimuli. The proportion of correct responses is plotted as a function of varying motion coherence as illustrated below the horizontal axis. The open and solid symbols represent psychophsyical and phsyiological data, respectively. (B) Psychometric functions measured with and without electrical stimulation of the neuron being recorded (solid and filled symbols, respectively). The preferred direction of the cell was upward, so the vertical axis plots the proportion of upward responses. In the absence of stimulation, an uncorrelated display (0% coherence) evoked 50% upward responses (chance performance). In the presence of stimulation, the function was shifted leftward so that an uncorrelated display evoked a significant proportion of upward responses.

of upward responses, and that a value of 0.5 represents equal upward and downward responses. In the absence of stimulation, the proportion of''upward'' responses increased above 0.5 only when the stimulus had a significant number of upwardly moving signal dots. In the presence of electrical stimulation the proportion of upward responses increased above 0.5 even when the signal dots were actually moving downward (negative values of correlation). In fact, electrical stimulation shifted the psychometric function leftward so that a smaller coherence level in the presence of stimulation produces a response associated with a larger coherence level in the absence of stimulation. This effect of stimulation, along with the lesion studies and the close correspondence between behavior and neural response, provides strong evidence that MT neurons are involved in the processing of large field motion stimuli.

Unraveling Alzheimers Disease

Unraveling Alzheimers Disease

I leave absolutely nothing out! Everything that I learned about Alzheimer’s I share with you. This is the most comprehensive report on Alzheimer’s you will ever read. No stone is left unturned in this comprehensive report.

Get My Free Ebook


Post a comment