Biomedical subjects

S D Elfar

Publications and source records attributed to S D Elfar.

3 recordsLinked to original sources

PubMed2000

Some transformations of color information from lateral geniculate nucleus to striate cortex.

We have recorded the responses of single cells in the lateral geniculate nucleus (LGN) and striate cortex of the macaque monkey. The response characteristics of neurons at these successive visual processing levels were examined with isoluminant gratings, cone-isolating gratings, and luminance-varying gratings. The main findings were: (i) Whereas almost all parvo- and konio-cellular LGN cells are of just two opponent-cell types, either differencing the L and M cones (L(o) and M(o) cells), or the S vs. L + M cones (S(o) cells), relatively few striate cortex simple cells show chromatic responses along these two cardinal LGN axes. Rather, most are shifted away from these LGN chromatic axes as a result of combining the outputs (or the transformed outputs) of S(o) with those of L(o) and/or M(o) cells. (ii) LGN cells on average process color information linearly, exhibiting sinusoidal changes in firing rate to isoluminant stimuli that vary sinusoidally in cone contrast as a function of color angle. Some striate cortex simple cells also give linear responses, but most show an expansive response nonlinearity, resulting in narrower chromatic tuning on average at this level. (iii) There are many more +S(o) than -S(o) LGN cells, but at the striate cortex level -S(o) input to simple cells is as common as +S(o) input. (iv) Overall, the contribution of the S-opponent path is doubled at the level of the striate cortex, relative to that at the LGN.

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PubMed2000

Spatial and temporal receptive fields of geniculate and cortical cells and directional selectivity.

The spatio-temporal receptive fields (RFs) of cells in the macaque monkey lateral geniculate nucleus (LGN) and striate cortex (V1) have been examined and two distinct sub-populations of non-directional V1 cells have been found: those with a slow largely monophasic temporal RF, and those with a fast very biphasic temporal response. These two sub-populations are in temporal quadrature, the fast biphasic cells crossing over from one response phase to the reverse just as the slow monophasic cells reach their peak response. The two sub-populations also differ in the spatial phases of their RFs. A principal components analysis of the spatio-temporal RFs of directional V1 cells shows that their RFs could be constructed by a linear combination of two components, one of which has the temporal and spatial characteristics of a fast biphasic cell, and the other the temporal and spatial characteristics of a slow monophasic cell. Magnocellular LGN cells are fast and biphasic and lead the fast-biphasic V1 subpopulation by 7 ms; parvocellular LGN cells are slow and largely monophasic and lead the slow monophasic V1 sub-population by 12 ms. We suggest that directional V1 cells get inputs in the approximate temporal and spatial quadrature required for motion detection by combining signals from the two non-directional cortical sub-populations which have been identified, and that these sub-populations have their origins in magno and parvo LGN cells, respectively.

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PubMed1989

Spatial-frequency organization in primate striate cortex.

We measured the spatial-frequency tuning of cells at regular intervals along tangential probes through the monkey striate cortex and correlated the recording sites with the cortical cytochrome oxidase (CytOx) patterns to address three questions with regard to the cortical spatial-frequency organization. (i) Is there a periodic anatomical arrangement of cells tuned to different spatial-frequency ranges? We found there is, because the spatial-frequency tuning of cells along tangential probes changed systematically, varying from a low frequency to a middle range to high frequencies and back again repeatedly over distances of about 0.6-0.7 mm. (ii) Are there just two populations of cells, low-frequency and high-frequency units, at a given eccentricity (perhaps corresponding to the magno- and parvocellular geniculate pathways) or is there a continuum of spatial-frequency peaks? We found a continuum of peak tuning. Most cells are tuned to intermediate spatial frequencies and form a unimodal rather than a bimodal distribution of cell peaks. Furthermore, the cells with different peak frequencies were found to be continuously and smoothly distributed across a module. (iii) What is the relation between the physiological spatial-frequency organization and the regions of high CytOx concentration ("blobs")? We found a systematic correlation between the topographical variation in spatial-frequency tuning and the modular CytOx pattern, which also varied continuously in density. Low-frequency cells are at the center of the blobs, and cells tuned to increasingly higher spatial frequencies are at increasing radial distances.

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