Biomedical subjects

Kenneth O Johnson

Publications and source records attributed to Kenneth O Johnson.

4 recordsLinked to original sources

PubMed2002

Receptive field structure in cortical area 3b of the alert monkey.

More than 350 neurons with fingerpad receptive fields (RFs) were studied in cortical area 3b of three alert monkeys. Random dot patterns, which contain all stimulus patterns with equal probability, were scanned across these RFs at three velocities and eight directions to reveal the RFs' spatial and temporal structure. Area 3b RFs are characterized by three components: (1) a single, central excitatory region of short duration, (2) one or more inhibitory regions, also of short duration, that are adjacent to and nearly synchronous with the excitation, and (3) a region of inhibition that overlaps the excitation partially or totally and is temporally delayed with respect to the first two components. As a result of these properties, RF spatial structure depends on scanning direction but is virtually unaffected by changes in scanning velocity. This RF characterization, which is derived solely from responses to scanned random-dot patterns, predicts a neuron's responses to random patterns accurately, as expected, but it also predicts orientation sensitivity and preferred orientation measured with a scanned bar. Both orientation sensitivity and the ratio of coincident inhibition (number 2 above) to excitation are stronger in the supra- and infragranular layers than in layer IV.

Action Potentials↗

PubMed2002

Synchrony: a neuronal mechanism for attentional selection?

Attentional selection involves brain processes that select and control the flow of information into the mechanisms that underlie perception and consciousness. One theory proposes that the neural activity that represents the stimuli or events to be attended to is selected through modification of its synchrony. Recent experimental evidence supports this theory, by showing that changes in attentional focus increase the synchrony of neural firing in some neuron pairs and decrease it in others.

Animals↗

PubMed2002

Spatial acuity after digit amputation.

Digit amputation in human and non-human primates results in reorganization of somatosensory cortex in which the representations of adjacent, intact digits expand to fill the cortical region previously devoted to the amputated digits. Whether this expanded representation results in improved sensory performance has not been determined. Consequently, we measured the ability to recognize small objects (raised letters) with a digit adjacent to the amputation and the same digit on the normal, contralateral hand in 15 amputees. The same digits were also tested in 15 age-matched, amputation-free subjects. There was no significant difference in recognition scores between digits in the amputees or between amputees and control subjects. More detailed analyses of specific confusion patterns and of the improvement with practice showed no significant differences. As far as we could determine, the cortical expansion that is presumed to accompany digit amputation had no effect on tactile pattern recognition performance.

Adult↗

PubMed2002

Neural coding and the basic law of psychophysics.

There have been three main ideas about the basic law of psychophysics. In 1860, Fechner used Weber's law to infer that the subjective sense of intensity is related to the physical intensity of a stimulus by a logarithmic function (the Weber-Fechner law). A hundred years later, Stevens refuted Fechner's law by showing that direct reports of subjective intensity are related to the physical intensity of stimuli by a power law. MacKay soon showed, however, that the logarithmic and power laws are indistinguishable without examining the underlying neural mechanisms. Mountcastle and his colleagues did so, and, on the basis of transducer functions obeying power laws, inferred that subjective intensity must be related linearly to the neural coding measure on which it is based. In this review, we discuss these issues and we review a series of studies aimed at the neural mechanisms of a very complex form of subjective experience-the experience of roughness produced by a textured surface. The results, which are independent of any assumptions about the form of the psychophysical law, support the idea that the basic law of psychophysics is linearity between subjective experience and the neural activity on which it is based.

Animals↗