Biomedical subjects

L J Toth

Publications and source records attributed to L J Toth.

5 recordsLinked to original sources

PubMed1997

Optically imaged maps of orientation preference in primary visual cortex of cats and ferrets.

Feature maps in the cerebral cortex constitute orderly representations of response features created within the cortex; an example is the mapping of orientation-selective neurons in visual cortex. We have compared the properties of orientation maps in area 17 of cats and ferrets, obtained by optical imaging of intrinsic signals. Orientation maps in both species contain a quasi-periodic distribution of iso-orientation domains that are organized into a lattice of pinwheels. However, the spatial density of orientation domains and of pinwheels in ferret area 17 is nearly twice that in cat area 17. The ferret map also contains more discontinuities, or fractures, where orientation changes abruptly. The size of orientation domains scales with interdomain spacing, so that the ratio of the two is approximately the same in both species. Consistent with this finding, the orientation tuning width of individual pixels is similar in the two. The magnitude of orientation preference, however, is much lower in ferret compared to cat. The greater incidence of fractures in ferret appears to be due to proportionately greater overlap between domains of different orientations, particularly along fracture lines that link pinwheel centers. We hypothesize that a key determinant of orientation maps, the relationship between orientation domain size and spacing, expresses an anatomical link between sizes of thalamocortical arbors and horizontal intracortical connections in area 17.

Animals↗

PubMed1997

Integration of local inputs in visual cortex.

In mammalian visual cortex, local connections are ubiquitous, extensively linking adjacent neurons of all types. In this study, optical maps of intrinsic signals and responses from single neurons were obtained from the same region of cat visual cortex while the effectiveness of the local cortical circuitry was altered by focally disinhibiting neurons within a column of known orientation preference. Maps of intrinsic signals indicated that local connections provide strong and functional subthreshold inputs to neighboring columns of other orientation preferences, altering the observed orientation preference to that of the disinhibited column. However, measuring the suprathreshold response using single-cell recordings revealed only mild changes of preferred orientation over the affected region. Because strongly tuned subthreshold inputs from cortex only marginally affect the tuning of a cortical cell's output, it is concluded that local cortical inputs are integrated weakly compared to geniculate inputs. Such circuitry potentially allows for the normalization of responses across a wide range of input activity through local averaging.

Animals↗

PubMed1996

Subthreshold facilitation and suppression in primary visual cortex revealed by intrinsic signal imaging.

Neurons in primary visual cortex (area 17) respond vigorously to oriented stimuli within their receptive fields; however, stimuli presented outside the suprathreshold receptive field can also influence their responses. Here we describe a fundamental feature of the spatial interaction between suprathreshold center and subthreshold surround. By optical imaging of intrinsic signals in area 17 in response to a stimulus border, we show that a given stimulus generates activity primarily in iso-orientation domains, which extend for several millimeters across the cortical surface in a manner consistent with the architecture of long-range horizontal connections in area 17. By mapping the receptive fields of single neurons and imaging responses from the same cortex to stimuli that include or exclude the aggregate suprathreshold receptive field, we show that intrinsic signals strongly reveal the subthreshold surround contribution. Optical imaging and single-unit recording both demonstrate that the relative contrast of center and surround stimuli regulates whether surround interactions are facilitative or suppressive: the same surround stimulus facilitates responses when center contrast is low, but suppresses responses when center contrast is high. Such spatial interactions in area 17 are ideally suited to contribute to phenomena commonly regarded as part of "higher-level" visual processing, such as perceptual "popout" and "filling-in."

Animals↗

PubMed1992

Effect of stimulus contrast and size on NMDA receptor activity in cat lateral geniculate nucleus.

1. We studied the effect of varying excitatory and inhibitory drive on the N-methyl-D-aspartate (NMDA) receptor-mediated component of the visual responses of neurons in the cat dorsal lateral geniculate nucleus (dLGN) by varying the contrast and size of stimuli presented to the receptive fields of these cells. 2. Cells were classified as either on- or off-center, X or Y, and lagged or nonlagged. Stimulus contrast, and hence the amount of excitatory drive, was varied by changing the brightness of a spot, whose size and location matched the cell's receptive field center, relative to a fixed background luminance. Responses to varying contrast were collected from each cell before, during, and after iontophoretic application of D-2-amino-5-phosphonovaleric acid (D-APV), a specific NMDA receptor antagonist. From each contrast-response plot, a sigmoidal curve fit yielded five parameters on which we examined the effect of D-APV: the threshold contrast, saturation contrast, contrast at half saturation (C50), slope (gain) at C50, and saturation response. 3. In most cells, application of D-APV reduced both the saturation response and the gain of the contrast-response curve, but did not reduce or change significantly the threshold contrast, saturation contrast, or C50. 4. Cells varied in their sensitivity to D-APV, but for any given cell, the D-APV-sensitive component was nearly always a linear function of the control visual response level. Thus, for a spot of optimal size, there was a constant proportion of the visual response attributable to NMDA receptors, regardless of the amplitude of the response. 5. When the effect of D-APV on the visual responses to an optimal spot at varying contrasts was compared among different classes of dLGN cells, the visual responses of lagged X cells were reduced to a greater extent than those of either nonlagged X cells or the combined population of nonlagged X and Y cells. 6. Stimulus size (spot diameter) was also varied systematically at a fixed contrast to vary the inhibitory drive to dLGN cells. As stimulus size was increased, the response first increased because of increased stimulation of the receptive field center and then decreased because of increasing amounts of surround inhibition. 7. The D-APV-sensitive component of individual cell responses was greater when the stimulus spot was less than or equal to optimal size than when the spot was larger. Thus the contribution of NMDA receptors to the visual response decreased with increasing surround inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗

PubMedDate not supplied

A local circuit approach to understanding integration of long-range inputs in primary visual cortex.

Integration of inputs by cortical neurons provides the basis for the complex information processing performed in the cerebral cortex. Here, we have examined how primary visual cortical neurons integrate classical and nonclassical receptive field inputs. The effect of nonclassical receptive field stimuli and, correspondingly, of long-range intracortical inputs is known to be context-dependent: the same long-range stimulus can either facilitate or suppress responses, depending on the level of local activation. By constructing a large-scale model of primary visual cortex, we demonstrate that this effect can be understood in terms of the local cortical circuitry. Each receptive field position contributes both excitatory and inhibitory inputs; however, the inhibitory inputs have greater influence when overall receptive field drive is greater. This mechanism also explains contrast-dependent modulations within the classical receptive field, which similarly switch between excitatory and inhibitory. In order to simplify analysis and to explain the fundamental mechanisms of the model, self-contained modules that capture nonlinear local circuit interactions are constructed. This work supports the notion that receptive field integration is the result of local processing within small groups of neurons rather than in single neurons.

Contrast Sensitivity↗