Biomedical subjects

E Zohary

Publications and source records attributed to E Zohary.

6 recordsLinked to original sources

PubMed1994

Correlated neuronal discharge rate and its implications for psychophysical performance.

Single neurons can signal subtle changes in the sensory environment with surprising fidelity, often matching the perceptual sensitivity of trained psychophysical observers. This similarity poses an intriguing puzzle: why is psychophysical sensitivity not greater than that of single neurons? Pooling responses across neurons should average out noise in the activity of single cells, leading to substantially improved psychophysical performance. If, however, noise is correlated among these neurons, the beneficial effects of pooling would be diminished. To assess correlation within a pool, the responses of pairs of neurons were recorded simultaneously during repeated stimulus presentations. We report here that the observed covariation in spike count was relatively weak, the correlation coefficient averaging 0.12. A theoretical analysis revealed, however, that weak correlation can limit substantially the signalling capacity of the pool. In addition, theory suggests a relationship between neuronal responses and psychophysical decisions which may prove useful for identifying cell populations underlying specific perceptual capacities.

Action Potentials↗

PubMed1994

Neuronal plasticity that underlies improvement in perceptual performance.

The electrophysiological properties of sensory neurons in the adult cortex are not immutable but can change in response to alterations of sensory input caused by manipulation of afferent pathways in the nervous system or by manipulation of the sensory environment. Such plasticity creates great potential for flexible processing of sensory information, but the actual effects of neuronal plasticity on perceptual performance are poorly understood. The link between neuronal plasticity and performance was explored here by recording the responses of directionally selective neurons in the visual cortex while rhesus monkeys practiced a familiar task involving discrimination of motion direction. Each animal experienced a short-term improvement in perceptual sensitivity during daily experiments; sensitivity increased by an average of 19 percent over a few hundred trials. The increase in perceptual sensitivity was accompanied by a short-term improvement in neuronal sensitivity that mirrored the perceptual effect both in magnitude and in time course, which suggests that improved psychophysical performance can result directly from increased neuronal sensitivity within a sensory pathway.

Animals↗

PubMed1993

Mechanisms of velocity constancy.

Human observers can compare the physical velocities of objects (cm/sec) moving at different distances quite well, although the objects' retinal velocities (deg/sec) may vary considerably. This perceptual ability is called velocity constancy. We conducted a number of experiments to investigate what mechanisms observers use to attain this constancy and if pure motion signals can also be matched according to their physical speeds. Subjects were asked to match the velocities of two moving stimuli presented at different viewing distances. The stimuli consisted of sparse random-dot kinematograms or drifting Julesz patterns. The subjects matched the true physical velocities of the stimuli provided that the two visual scenes contained identical size references. Knowledge of the actual viewing distances proved to be irrelevant for evaluating the physical velocities of the stimuli. We conclude that velocity constancy is based upon a relative scaling algorithm.

Algorithms↗

PubMed1992

Population coding of visual stimuli by cortical neurons tuned to more than one dimension.

Neurons in the visual cortex are typically selective to a number of stimulus dimensions. Thus, there is a basic ambiguity in relating the response level of a single neuron to the stimulus values. It is shown that a multi-dimensional stimulus may be coded reliably by an ensemble of neurons, using a weighted average population coding model. Each neurons' contribution to the population signal for each dimension is the product of its response magnitude and its preferred value for that dimension. The sum of the products was normalized by the sum of the ensemble responses. Simulation results show that the representation accuracy increases as the square root of the number of units irrespective of the number of dimensions. Comparison of a specific 2D case of this population code for orientation and spatial frequency to behavioral discrimination levels yields that 10(3)-10(4) neurons are needed to reach psychophysical performance. Introduction of each additional dimension requires about 1.7 times the number of neurons in the ensemble to reach the same level of accuracy. This result suggests that neurons may be selective for only 3 to 5 dimensions. It also provides another rationale for the existence of parallel processing streams in vision.

Animals↗

PubMed1990

Time course of perceptual discrimination and single neuron reliability.

The reliability of identification of a visual target increases with time available for inspection of the stimulus. We suggest that the neural basis of this improvement is the existence of a mechanism for integrating a noisy firing rate over some period, leading to a reduction in mean firing rate variance with available processing time. We have determined the experimental time course of the improvement in reliability in a parallel search task where the available inspection time is limited by the presentation of a mask at various times after a brief stimulus. We compare the resulting psychometric functions with the predictions of a model based on Signal Detection Theory. The model is based on the assumption that the reliability of the observer's response is limited by the variability of the responses of individual neurons. The reliability of the discrimination between two stimuli at the neuronal level is then directly related to the ratio of the difference between their integrated mean responses (over many trials) to the response standard deviation. This reliability increases with inspection time. To demonstrate application of the model to electrophysiological data, "neurometric functions" are derived from the firing rates of a monkey V1 cortical neuron. The data were obtained while the animal was active in a discrimination task. The results correspond qualitatively to our observed human psychometric functions.

Animals↗

PubMed1989

How serial is serial processing in vision?

Visual search for an element defined by the conjunction of its colour and orientation has previously been shown to be a serial processing task since reaction times increase linearly with the number of distractor elements used in the display. Evidence is presented that there are parallel processing constituents to this serial search. Processing time depended on the ratio of the number of the two distractor types used, suggesting that only one type was scanned. Which type was scanned also depended on the distractor ratio, indicating that this decision was made after stimulus presentation and was based on a parallel figure-ground separation of the stimulus elements. Furthermore, in accordance with this serial scanning model, there was an increase in processing speed (elements scanned per second) with increase in number of elements to be scanned. This increased efficiency suggests that clumps of elements were processed synchronously. Under the stimulation conditions used, clumps contained six to sixteen elements and each clump was processed in 50-150 ms.

Attention↗