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Biomedical subjects

B Sakitt

Publications and source records attributed to B Sakitt.

18 recordsLinked to original sources
PubMed1983

The information transmitted at final position in visually triggered forearm movements.

Visually triggered forearm movements were analyzed by an Information Theory approach. Human subjects made smooth movements which were characterized by moderate speeds, ranging about 100 degrees per second, by continuity in the position and velocity traces, and attainment of final average EMG levels before completion of the movement. We calculated the information transmitted by final position, biceps EMG, triceps EMG, and the ratio of the EMGs. The results were: (1) The information transmitted by final joint angle increased with number of targets but gradually levelled off. The maximum value was slightly over 3 bits, corresponding to an equivalent number of less than nine independent arm positions for a single movement. (2) The information transmitted by the ratio of the EMGs exceeds that transmitted by the biceps or triceps alone. (3) A previous theoretical prediction based on a spring model (Sakitt, 1980a) gives a moderately good fit to the experimental EMG ratio as a function of final position over a large range of angles. Our results lend consistency to two ideas about the nature of visually triggered forearm movements. First, our finding about the EMG ratio suggests that the basic motor program for final position is probably in terms of relative allocation of innervations, rather than looking up individual values. Second, single movements of this kind transmit surprisingly little information. If this is the case, it suggests that very fine accuracy is not achieved by a single program but requires feedback in order to program and execute additional movement.

Forearm↗
PubMed1982

A model for the economical encoding of the visual image in cerebral cortex.

We propose a model for the first stage of the cortical transformation of the visual image based on the principle that the cortex encodes the information with the minimum number of channels mathematically needed. We restrict our model to be consistent with the data on size adaptations, the known relationships of acuity and the inverse of magnification factor with eccentricity, and the electrophysiological findings on the physiological uniformity of the striate cortex. Assuming that each hypercolumn analyzes a limited spatial domain, we apply the sampling theorem to show that only 16 channels, composed of 4 sizes, are needed for dimension. The extension to 2 dimensions leads to a possible scheme for the number, spacing, and orientational disposition of the elements, together with predictions about the number of inputs from the eyes and the total number of hypercolumns. Since all these predictions are consistent with physical and neural estimates, we conclude that the cortex may analyze the image along the lines we suggest.

Cerebral Cortex↗
PubMed1982

Why the cortical magnification factor in rhesus can not be isotropic.

Using the data of Hubel and Wiesel (1974) J. comp. Neurol. 158, 295-306, on macaque, the shape of the operculum of striate cortex (area 17) is calculated from their magnification factor values assuming that the cortical magnification factor is locally isotropic with respect to displacement on the cortical surface (from a fixed point of origin). Using geometric considerations, the shape predicted from the isotropic assumption is inconsistent with the actual shape given by anatomical reconstruction by LeVay, Hubel and Wiesel (1975) J. comp. Neurol. 159, 559-576. Rather the true shape is more consistent with the hypothesis that on the average the magnification factor along a meridian is twice the value as orthogonal to it. Except in the region of the horizontal meridian, this is often the same as assuming that the magnification factor across ocular dominance columns is twice that going parallel to them.

Animals↗
PubMed1980

A spring model and equivalent neural network for arm posture control.

A model is introduced for the motor program which controls final position. The first part of the model relates the biomechanical properties of the muscles to the EMG activities of the extensor and flexor muscles and thereby generates quantitative predictions for the relationships between the EMGs, final position, external forces, muscle stiffness, and muscle tension. To the extent that comparable data exist, the model is shown to give correct quantitative predictions. When only qualitative comparisons can be made, the model is consistent with the data in the literature. The model is complete and can be tested quantitatively in detail in the future. An equivalent circuit for the neural network that innervates the muscles is given. It is shown to have the advantages of making the programming of final position simple to either compute or look up in a table. In addition, new situations, such as adapting to a force, or an unusual viewing angle, lead to very simple changes in the basic program in terms of the equivalent circuit.

Arm↗
PubMed1980

The retinal basis of iconic memory: Eriksen and Collins revisited.

The parameters of luminance, duration, wavelength, and interstimulus interval (ISI) were varied on the successive-field task first employed by Eriksen and Collins in 1967 and 1968. This task requires observers to combine visually the information presented in two brief successive presentations for successful trigram recognition. The results of the luminance and duration manipulations indicate that as stimulus energy is increased, iconic memory also increases. The wavelength and ISI manipulations reveal that performance at 20 msec ISI reflects cone (photopic) sensitivity but that performance at 250 msec ISI reflects rod (scotopic) sensitivity. These findings are interpreted in support of the retinal conception of iconic memory, with the cones providing a very transient icon and the rods providing the bulk of the persistence.

Female↗
PubMed1979

Spare the rod and spoil the icon.

Short-term visual storage was investigated with a successive field paradigm, so that correct performance depended upon combining visual information from two targets that were never on simultaneously. In the first two experiments, the stimuli consisted of two slides, each containing a 10' red dot on a gray surround, and separated by an interstimulus interval (ISI) from 20 to 400 msec. Subjects had to determine if the dots were vertically or horizontally aligned. In Experiment 1, the stimuli had either no contrast for the rods or no luminance contrast for the cones or high contrast. At short ISIs the cone contrast determined performance, whereas at long ISIs the rod contrast determined performance. In fact, when the dots were invisible to the rods, the task was impossible for long ISIs. In Experiment 2, performance was compared for zero log rod contrast and for small departures from zero. Even a small departure from zero log rod contrast resulted in above-chance performance. In Experiment 3, the stimuli were luminous rectangles and the task was to decide whether or not a 4' spatial gap was present between the two successively presented rectangles. Wavelength, luminance, and ISI were varied under both photopic and scotopic adapting conditions. The result was that the rods performed the task for ISIs of 150 msec or longer under scotopic conditions and under the photopic conditions that we were able to test. The results of the experiments taken together are consistent with the hypothesis that the cone icon is short, whereas the rod icon is robust and long lasting.

Color Perception↗
PubMed1976

Iconic memory.

Explore the source record for details and available documents.

Afterimage↗
PubMed1975

Locus of short-term visual storage.

A rod monochromat can preserve visual information in iconic memory even when the initial stimulus is invisible to the subject. Since the initial invisibility is due to rod saturation, it can be shown that all the information must have been stored inside the photoreceptors. Because the spectral sensitivity for producing icons in normal subjects is that of the rods, the conclusion is that in normal subjects, under ordinary viewing conditions, the photoreceptors are the primary store for iconic memory.

Female↗
PubMed1972

Counting every quantum.

1. Human subjects were asked to rate both blanks and very dim flashes of light under conditions of complete dark adaptation at 7 degrees in the periphery. The ratings used were 0, 1, 2, 3, 4, 5, and 6.2. For one subject (B.S.) the distributions of ratings were approximately Poisson distributions. The data were consistent with each rating being the actual number of effective quantal absorptions plus the number of noise events. This subject was presumably able to count every rod signal (effective absorptions plus noise).3. For two other subjects, the data were consistent with the ratings being one less (L.F.) and two less (K.D.) than the number of effective absorptions plus noise. They were able to count every rod signal beginning with 2 and 3 respectively. A fourth subject's erratic data could not be fitted.4. The fraction of quanta incident at the cornea that resulted in a rod signal was estimated to be about 0.03 which is consistent with physical estimates of effective absorption for that retinal region.5. A simulated forced choice experiment leads to an absolute threshold about 0.40 log units below the normal yes-no absolute threshold. This and other results indicate that subjects can use the sensory information they receive even when only 1, 2 or 3 quanta are effectively absorbed, depending on the individual. Humans may be able to count every action potential or every discrete burst of action potentials in some critical neurone.

Dark Adaptation↗
PubMed1971

Configuration dependence of scotopic spatial summation.

1. Psychophysical experiments were done under conditions of complete dark adaptation, 7 degrees in the human temporal retina, using blue-green flashes brief enough to be within the range of complete temporal summation.2. Frequency-of-seeing experiments were done for two 3.4' squares separated by 20' or 23' and for each square separately. The results were consistent with the hypothesis that the squares acted independently in determining the absolute threshold. There was no observable summation occurring between them.3. The absolute threshold (50% seeing) for two 3.4' squares (the centre-to-centre separation varied from 3.4' to 56') was measured relative to the absolute threshold for just one such square. It required more photons at the cornea at threshold for two spots than for one such spot, except when they were actually touching.4. The same subject, in the same retinal region and under otherwise identical test conditions, showed complete spatial summation at absolute threshold for circular spots up to 30' in diameter.5. Similar experiments were done on other subjects using 3.4' squares and lines of width 3.4'. All subjects showed complete summation at absolute threshold for lines 34' long. All showed lack of spatial summation for two squares unless the two squares touched to form a rectangle.6. The absolute threshold for N small spots was compared to the absolute threshold for 1 spot, where N varied from 2 to 25. Again complete spatial summation broke down even when all the spots lay within the Ricco area.7. It was shown that the results are inconsistent with the hypothesis of a linear output and a linear summation of excitations.8. Various hypotheses are examined and it is shown that no simple hypothesis can explain all the results. The conclusion is that spatial summation at absolute threshold is configuration dependent.

Adaptation, Ocular↗