Hugo Maes

Factors influencing velocity coding in the human visual system.

Differential velocity detection in the fovea was measured over a wide range of velocities (0.25-256 degrees/sec). Differential thresholds were minimum (about 6%) for intermediate velocities (4-32 degrees/sec). Velocity judgements were shown not to depend on duration judgments. The U-shaped curve relating differential velocity detection and velocity was preserved at different background levels and different contrasts. The physiological correlates of these observations are discussed.

The Speed Tuning of Medial Superior Temporal (Mst) Cell Responses to Optic-Flow Components

The responses of macaque medial superior temporal (MST) cells to translation and to the optic-flow components—rotation, expansion/contraction, and deformation—were examined with particular regard to the speed tuning of MST cells for optic-flow stimuli and the effect of removing speed gradients from those stimuli. The use of position invariance as an indispensable criterion for assessing the authenticity of responses to optic flow is reviewed. By extending the scope of testing to include higher speeds it is found, in contrast to in previous reports, that MST cells generally respond to optic-flow components with a speed—response profile which is tuned for a particular range of speeds. Removal of the speed gradient had little effect on this observation. These and other properties of MST cells lead to the conclusion that one of the major functions of MST is the detection and encoding of self-motion.

Genetic variance of adolescent growth in stature

The aim of this study is to quantify the genetic and environmental variation in biological parameters of the adolescent growth in stature in both sexes, using structural equation models. Standing height was measured in 99 twin pairs from the Leuven Longitudinal Twin Study. Subjects were seen at semi-annual intervals between 10 and 16 years and at 18 years. Preece-Baines model I was fitted to the individual data and biologically meaningful parameters were derived. Path models were fitted to these parameters and a gender heterogeneity analysis was performed. A model including additive genetic and specific environmental factors (AE-model), allowing for a difference in total variance or in genetic/environmental variance components in males and females best explains most of the growth characteristics. For the timing and velocity of the adolescent growth spurt, no gender heterogeneity was observed, and the genetic (0.89 to 0.93) and specific environmental (0.07 to 0.11) contributions were equal in both sexes. For stature at take-off, stature at peak height velocity and for adult stature, gender heterogeneity was observed and different models appeared to result in the best fit for boys and girls. For height at PHV and adult stature, the significant contribution of a common environmental (0.39 to 0.56) factor in boys was noteworthy. For percentage of adult stature and distances between relevant time points of the growth curve, a non-scalar effect was observed, indicating that different genes affect these characteristics in boys and girls. It is concluded that most characteristics of the growth spurt in stature are under strong genetic control. Gender heterogeneity is present for stature and for a number of derived growth characteristics.The aim of this study is to quantify the genetic and environmental variation in biological parameters of the adolescent growth in stature in both sexes, using structural equation models. Standing height was measured in 99 twin pairs from the Leuven Longitudinal Twin Study. Subjects were seen at semi-annual intervals between 10 and 16 years and at 18 years. Preece-Baines model I was fitted to the individual data and biologically meaningful parameters were derived. Path models were fitted to these parameters and a gender heterogeneity analysis was performed. A model including additive genetic and specific environmental factors (AE-model), allowing for a difference in total variance or in genetic/environmental variance components in males and females best explains most of the growth characteristics. For the timing and velocity of the adolescent growth spurt, no gender heterogeneity was observed, and the genetic (0.89 to 0.93) and specific environmental (0.07 to 0.11) contributions were equal in both sexes. For stature at take-off, stature at peak height velocity and for adult stature, gender heterogeneity was observed and different models appeared to result in the best fit for boys and girls. For height at PHV and adult stature, the significant contribution of a common environmental (0.39 to 0.56) factor in boys was noteworthy. For percentage of adult stature and distances between relevant time points of the growth curve, a non-scalar effect was observed, indicating that different genes affect these characteristics in boys and girls. It is concluded that most characteristics of the growth spurt in stature are under strong genetic control. Gender heterogeneity is present for stature and for a number of derived growth characteristics.

Visual cortical correlates of visible persistence

In order to evaluate their possible role in visible persistence, cortical cells from area 17 of the cat were investigated with a stationary light bar flashed for different durations. Thirteen out of 72 cells with non-overlapping On and Off subregions were able to respond to the briefest On stimulus (12.5 msec) for low and moderate contrasts. The responses of these cells outlasted brief On stimuli and this neural persistence increased as the On duration was shortened, mimicking the inverse duration effect of visible persistence. The 30 cells with overlapping On and Off subregions were all able to respond to brief stimuli but their neural persistence was independent of stimulus duration. At very high contrast levels, the inverse duration effect, observed in cells with non-overlapping subregions, disappeared since the On responses were followed by Off rebound discharges regardless of stimulus duration. It is suggested that the latter responses are a possible cortical equivalent of positive afterimages.

Velocity discrimination in central and peripheral visual-field

Just-noticeable differences (jnds) in velocity were measured as a function of reference velocity for central and peripheral vision. The velocity discrimination curves plotting jnds in velocity, expressed as Weber fractions, as a function of reference velocity were U shaped at all eccentricities. Under almost every stimulus condition the increase in jnd in velocity with increasing eccentricity was significantly larger at low reference velocities than at high reference velocities. Consequently the shift toward higher velocities with increasing eccentricity was much clearer for the lower end of the velocity-discrimination curve than for the upper end. These results are in agreement with the predictions derived from the response characteristics of velocity-tuned cells. Control experiments involving direction discrimination have shown that the impossibility of making fine velocity judgments at high speeds is due not to too weak a contrast for the stimulus motion to be visible but to a limitation in the neural apparatus analyzing velocity.

Functional changes across the 17–18 border in the cat

SummaryChanges in velocity sensitivity, receptive field (RF) position, and RF size were investigated in long oblique penetrations crossing the 17–18 border. The penetrations were histologically reconstructed and the border determined by cytoarchitectonics. In cortex subserving central and paracentral vision change in velocity sensitivity allowed a reasonable physiological identification of the 17–18 border. The physiological border correlates well with the histological border zone, best with its medial edge. Changes in RF position and RF size are of little use for physiological identification of the border in this region. In this cortical region area 18 representation of the vertical meridian (VM) has a high magnification factor. In cortex subserving peripheral vision, the change in velocity sensitivity was small and the change in RF position coincided with the cytoarchitectonics.

Temporal integration in cat visual cortex: A test of bloch's law

Some units in the cat visual cortex fail to respond to a briefly flashed bar and it has been suggested that such neurons function as visual integrators with a long time constant. To test this integrator hypothesis, a study was made using presentations of a bar, flashed over the receptive field for various durations and at different luminances. Some cortical cells indeed showed an increase in the time to peak latency and in the response amplitude when stimulus duration was prolonged up to 320 msec. Such units obeyed Blochs law for durations over 100 msec.

STIMUL: stimulus control and multihistogram analysis of single neurone recordings

A higher-level computer language (STIMUL) is described. It has been developed for the control and analysis of single or multiple neurone recording experiments with a large number of complex (visual) stimulus conditions. The main features of this language are: (i) its syntax closely resembles FORTRAN and BASIC, (ii) stimulus conditions can be presented in a random order, generating multiple interleaved post-stimulus time histograms, (iii) an experiment can be described in advance but experimental parameters or strategy are easy to modify on-line, (iv) all relevant analyses of the data can be performed, displayed and plotted both off and on-line, in the latter case without disturbing the experimental flow.

Visual evoked potentials elicited by a moving unidimensional noise pattern

(1) Motion onset and offset visual evoked potentials (VEPs) were recorded in normal human subjects using a unidimensional noise pattern moving at 1, 8 and 64 degrees/s. The maximum N1-P1 amplitude of the motion onset response was obtained when using a fine noise pattern (maximum energy at 5.2 cpd) moving at 8 degrees/s. (2) At a velocity of 8 degrees/s, the motion onset response (fine pattern, 0.70 contrast) showed a morphology similar to the pattern disappearance response. Both at a lower (1 degrees/s) and a higher velocity (64 degrees/s) the N1-P1 amplitude of the motion onset complex was significantly reduced. The latency of the motion onset response (8 degrees/s) and the pattern disappearance complex were significantly different. (3) The effect of lowering the spatial content of the noise pattern on the amplitude of the motion onset response was different for the 3 velocities tested: the largest effect was at the lower velocity of 1 degrees/s; there was no similar effect on the pattern disappearance response. (4) With decreasing contrast, the N1-P1 amplitude of the motion onset response at 8 degrees/s decreased, but this reduction in amplitude was much less than that of the disappearance response. The contrast dependency of the motion onset complex was identical for binocular and monocular recordings. (5) Increasing the motion duration or the duration of the interstimulus interval did not alter the general morphology of the motion response.

Time course of feeding induced by pentobarbital-injections into the rat's VMH☆

Chemical blockade of the ventromedial hypothalamus (VMH) by single microinjections of pentobarbital in ad lib fed male Wistar rats induced a meal that did not outlast a 10-min postinjection period. When food was presented only after this period, feeding was nearly absent. Repeating the injection after 10 and 20 min led each time to eating, although the amount consumed decreased. Scanning behaviour was drastically depressed during the 4-min postinjection period but then recovered within 6-11 min. Sniffing into the litter was enhanced for about the same interval (8-10 min). Fixed-ratio lever pressing for food upon pentobarbital-blockade of the VMH also tended to disappear after this period, whether many (low ratios) or few (high ratios) food pellets were obtained. The limited meal size after VMH-blockade therefore presumably resulted from the waning of the anaesthetic effect (e.g. diffusion) in this brain site.