Search PubMed⌕ Search

Biomedical subjects

M J Morgan

Publications and source records attributed to M J Morgan.

At least 163 records · Page 9Linked to original sources
PubMed1983

On the failure of spatiotemporal interpolation: a filtering model.

A vernier target moving discontinuously between discrete spatial stations is seen as having a spatial offset if its component bars are presented at spatially aligning stations with a temporal delay (the interpolation effect). In agreement with previous work, we find that the threshold for detecting this virtual spatial offset is the same as that for detecting an explicit spatial offset, provided that certain spatial and temporal constraints are met. Interpolation is a completely efficient process provided that the spatial interval between stations in the movement trajectory is less than 3-4 min arc, but thereafter it shows a gradual decline in efficiency. This paper presents a model of the decline in efficiency which arises, we suggest, not because of a failure of sampling to represent the original stimulus, but because of a progressive mismatch between the sampled signal and the bandwidth of the spatial filters involved in interpolation. Our model is compared with the constant velocity model of Fahle & Poggio [Proc. R. Soc. Lond. B 213, 451-477 (1981)].

Humans↗
PubMed1983

Mechanisms responsible for the assessment of visual location: theory and evidence.

Vernier acuity demonstrates a fine sense of relative visual location, which can be exploited to examine the question of mechanisms for the extraction of position information. By determining the appearance and location of composite stimuli consisting of a number of unresolved parallel bars of different luminances, we have demonstrated that vernier acuities of the order of 5 sec arc are due to mechanisms with an inflexible manner of encoding local intensity changes in the retinal image. The results are compared and contrasted with current theoretical approaches to the problems of early visual coding, and are best described by a model which encodes only the occurrence and location of zero-crossings in the second derivative of the retinal light distribution.

Humans↗
PubMed1983

Exposure duration affects the sensitivity of vernier acuity to target motion.

Threshold vernier acuity was measured under different conditions of target movement and exposure duration. In the case of a simple two-line vernier target, image motion up to about 3 deg/sec had little effect upon threshold for a briefly exposed (150 msec) target, which is relatively poor even for a stationary stimulus, but produced a decrement in acuity for a continuously exposed stimulus. This finding was repeated in a second experiment, which used a centroid cue to vernier offset, and which compared the effects of horizontal and vertical target orientation. It is suggested that image motion and reduced exposure duration restrict the proportion of the light spread function that can be usefully sampled by the neural networks responsible for hyperacuity.

Differential Threshold↗
PubMed1983

The recognition and representation of edge blur: evidence for spatial primitives in human vision.

Experiments are reported that measure the precision with which observers can (a) discern the degree of blur of an edge, (b) distinguish between two different types of blur, and relate the locations of two such edges by judging their (c) misalignment or (d) separation. Three blurring functions were used, and detailed examination of the retinal stimulus in each case leads to a simple hypothesis for the visual coding of edge blur. This is interpreted in terms of a general and primitive code for the transduction of local and widespread spatial luminance changes, in which stationary points in an approximation to the second derivative are detected and localized. The phenomena of illusory brightness changes in Mach bands and the Chevreul illusion find a ready explanation in this theory.

Discrimination, Psychological↗
PubMed1983

Mental rotation: a computationally plausible account of transformation through intermediate steps.

A critical difficulty in theories of the 'mental rotation' phenomenon has been to find a computationally plausible reason why the rotation should occur in small intermediate steps. It is pointed out that this difficulty is peculiar to metrical representations: if spatial relations are presented symbolically but nonmetrically, then the iterative or recursive application of minimal transformations is memory saving. A program ROTTER is described to illustrate this principle.

Discrimination, Psychological↗
PubMed1983

Stimulus features that determine the visual location of a bright bar.

A modification to the standard vernier target that has a detrimental effect on acuity is described. The addition of an extra bar alongside one of the test bars and directly underneath the other increases thresholds by an amount that is a monotonic function of its luminance. This allows for the hypothesis that the location of a bright bar is a function of some widespread description of the light distribution arising from such a bar on the retina, rather than some local feature of such a distribution. In particular, the data are not consistent with any simple notion of boundary extraction and support the conjecture that position of a bar is assigned to the mean or centroid of its light distribution.

Form Perception↗
PubMed1982

The regulation of carbohydrate metabolism in animal cells: isolation of starch- and maltose-utilizing variants.

Chinese-hamster ovary (CHO-K1) cells can grow in medium in which glucose is replaced by either maltose, trehalose, or starch. This growth is due to the presence in the medium of serum enzymes which degrade these substrates to glucose. When the enzymes are inactivated by heat treatment, the cells no longer grow on these substrates but still grow on glucose. Medium containing heat-inactivated serum and the appropriate substrate in place of glucose has been successfully used to isolate starch- and maltose-utilizing variants of CHO-K1 cells.

Animals↗
PubMed1982

Hyperacuity for luminance phase angle in the human visual system.

A moving target with identical trajectories in the two eyes appeared shifted in depth if flickered with a phase difference between the eyes. The direction of the depth shift was the same as if the phase-lagging eye had been seeing the target with a spatial advance in the direction of target motion. Maximum acuity for this effect was in the region of 7 deg of phase angle, which may be expressed as a spatial difference between corresponding luminance points of about 13 sec arc. Acuity was approximately constant over a wide range of modulation periods from 50 to 150 msec when expressed in terms of phase angle, but fell off rapidly at higher and at lower frequencies. The results cannot be explained simply by either conventional disparity theory or by spatio-temporal interpolation, but suggest that inter-ocular correspondence is influenced by dynamic luminance information.

Depth Perception↗
PubMed1982

The modulation transfer function of a display oscilloscope: measurements and comments.

It is pointed out that "spot size" of high resolution oscilloscope displays typically refers to the width of the spot at its half peak amplitude point, not to the width of the whole light distribution. The modulation transfer function of oscilloscope displays can have a more pronounced effect upon stimulus contrast than is generally realized, particularly when short viewing distances are used. We present measures of the spot profiles of a typical display, and use these to derive the MTF of the instrument. A simple correction is described to derive the actual contrast of a sine wave grating, given the manufacturer's specification of spot size.

Animals↗
PubMed1982

Effect of motion sweep duration and number of stations upon interpolation in discontinuous motion.

Spatial and temporal vernier offset thresholds were determined for a target moving in discrete spatial steps between stations. In the spatial case, the two bars comprising the target had a relative spatial offset at each station but were synchronous. In the temporal case the two bars appeared at aligning stations with a temporal asynchrony. Temporal thresholds were converted into spatial units by calculating the distance the target would have travelled during the temporal delay. When compared in this way, temporal and spatial thresholds were very similar if the duration of the motion sequence was 300 msec or greater, but at briefer durations temporal thresholds became progressively higher. The possibility that eye pursuit might account for these data was ruled out by recording eye movements and rejecting trials on which tracking occurred. Analysis of eye records revealed the surprising fact that some tracking can occur even when the duration of the motion sequence is as little as 150 msec and random in direction.

Eye Movements↗
PubMed1981

Regulation of carbohydrate metabolism in cultured mammalian cells: energy provision in a glycolytic mutant.

The metabolism of radioactively labelled D-glucose, L-glutamine, and L-glutamate has been determined in a glycolytic mutant of Chinese-hamster ovary cells, R1.1.7, and in its parent, CHO-K1. The complete oxidation of glucose via the TCA-cycle is negligible in both cell types, but there is significant oxidation of carbon-1. CHO-K1 cells derive most of their energy from glycolysis and are independent of respiration in the short term. R1.1.7 cells are respiration-dependent and are rapidly killed by respiratory inhibitors. Both cell types oxidize L-glutamine and L-glutamate, but oxidation of these substrates does not appear sufficient to satisfy completely the energy requirements of R1.1.7 cells.

Anemia, Hemolytic, Congenital Nonspherocytic↗
PubMed1981

Vernier acuity and stereopsis with discontinuously moving stimuli.

Apparent stereoscopic disparities or vernier displacements in moving stimuli can be introduced by temporal delays as well as by conventional spatial shifts. A method is described for observing these effects in dynamic dot patterns containing motion sequences of determinate length. The implications for theories of stereoscopic and vernier acuity are discussed.

Depth Perception↗
PubMed1980

Role of alcohol in the aetiology of hypertension.

Hypertension is a common condition facing medical practitioners on a day-to-day basis, yet much is to be learnt regarding its cause. Excess alcohol intake has been recognized by clinicians as a causal factor on an anecdotal basis for many years now. Little evidence for this has been provided from statistical analysis of formal studies. On this basis, a study was undertaken in a population of 85 adult males to see if any correlations could be drawn between alcohol intake and standard blood pressure readings, taking into account as many other variables as possible which may contribute to blood pressure, such as age, body weight and smoking habits. In this study, a statistically significant relationship between alcohol intake and blood pressure is demonstrated, indicating a need for further investigation in this field.

Adult↗
PubMed1980

Analogue models of motion perception.

An object moving in discrete spatial jumps is difficult to distinguish from a continuously moving object, provided the time between jumps is not too great. The extent of this perceived continuity may be measured by probing the perceived spatial location at times between the target jumps, by either a vernier alignment or a stereoscopic technique. As the time between jumps increase the accuracy of spatial interpolation falls, until finally the object is seen only at its actual spatial locations. These results can be analysed in the frequency domain by treating the signal for apparent motion as the analogue of a periodic waveform containing relatively low frequencies (the continuous motion) and higher frequencies giving rise to the discreteness of the motion. If such an input has the higher frequencies progressively removed by physical filtering, it is perceived ads increasingly continuous. The fact that such filtering is not necessary for perceived continuity when the discrete jumps occur at rates greater than about 30 Hz suggests that frequencies greaster than that limit are removed by the visual system itself.

Cognition↗