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B Jagadeesh

Publications and source records attributed to B Jagadeesh.

10 recordsLinked to original sources
PubMed2005

Molecular dynamic heterogeneity of confined lipid films by 1H magnetization-exchange nuclear magnetic resonance.

The molecular dynamic heterogeneity of monolayer to submonolayer thin lecithin films confined to submicron cylindrical pores were investigated by 1H magnetization exchange nuclear magnetic resonance. In this experiment a z-magnetization gradient was generated by a double-quantum dipolar filter. The magnetization-exchange decay and buildup curves were interpreted with the help of a theoretical model based on the approximation of a one-dimensional spin-diffusion process in a three-domain morphology. The dynamic heterogeneity of the fatty acid chains and the effects of the surface area per molecule, the diameter of the pores, and the temperature were characterized with the help of local spin-diffusion coefficients. The effect of various parameters on the molecular dynamics of the mobile region of the fatty acid chains was quantified by introducing an ad hoc Gaussian distribution function of the 1H residual dipolar couplings. For the lipid films investigated in this study, the surface induced order and the geometrical confinement affect the chain dynamics of the entire molecule. Therefore, each part of the chain independently reflects the effect of surface coverage, pore size, and temperature.

Aluminum Oxide↗
PubMed2001

Learning increases stimulus salience in anterior inferior temporal cortex of the macaque.

With experience, an object can become behaviorally relevant and thereby quickly attract our interest when presented in a visual scene. A likely site of these learning effects is anterior inferior temporal (aIT) cortex, where neurons are thought to participate in the filtering of irrelevant information out of complex visual displays. We trained monkeys to saccade consistently to one of two pictures in an array, in return for a reward. The array was constructed by pairing two stimuli, one of which elicited a good response from the cell when presented alone ("good" stimulus) and the other of which elicited a poor response ("poor" stimulus). The activity of aIT cells was recorded while monkeys learned to saccade to either the good or poor stimulus in the array. We found that neuronal responses to the array were greater (before the saccade occurred) when training reinforced a saccade to the good stimulus than when training reinforced a saccade to the poor stimulus. This difference was not present on incorrect trials, i.e., when saccades to the incorrect stimulus were made. Thus the difference in activity was correlated with performance. The response difference grew over the course of the recording session, in parallel with the improvement in performance. The response difference was not preceded by a difference in the baseline activity of the cells, unlike what was found in studies of cued visual search and working memory in aIT cortex. Furthermore, we found similar effects in a version of the task in which any of 10 possible pairs of stimuli, prelearned before the recording session, could appear on a given trial, thereby precluding a working memory strategy. The results suggest that increasing the behavioral significance of a stimulus through training alters the neural representation of that stimulus in aIT cortex. As a result, neurons responding to features of the relevant stimulus may suppress neurons responding to features of irrelevant stimuli.

Action Potentials↗
PubMed2000

Clustering of perirhinal neurons with similar properties following visual experience in adult monkeys.

The functional organization of early visual areas seems to be largely determined during development. However, the organization of areas important for learning and memory, such as perirhinal cortex, may be modifiable in adults. To test this hypothesis, we recorded from pairs of neurons in perirhinal cortex of macaques while they viewed multiple complex stimuli. For novel stimuli, neuronal response preferences for pairs of nearby neurons and far-apart neurons were uncorrelated. However, after one day of experience with the stimuli, response preferences of nearby neurons became more similar. We conclude that specific visual experience induces development of clusters of perirhinal neurons with similar stimulus preferences.

Action Potentials↗
PubMed1997

Direction selectivity of synaptic potentials in simple cells of the cat visual cortex.

Direction selectivity of synaptic potentials in simple cells of the cat visual cortex. J. Neurophysiol. 78: 2772-2789, 1997. The direction selectivity of simple cells in the visual cortex is generated at least in part by nonlinear mechanisms. If a neuron were spatially linear, its responses to moving stimuli could be predicted accurately from linear combinations of its responses to stationary stimuli presented at different positions within the receptive field. In extracellular recordings, this has not been found to be the case. Although the extracellular experiments demonstrate the presence of a nonlinearity, the cellular process underlying the nonlinearity, whether an early synaptic mechanism such as a shunting inhibition or simply the spike threshold at the output, is not known. To differentiate between these possibilities, we have recorded intracellularly from simple cells of the intact cat with the whole cell patch technique. A linear model of direction selectivity was used to analyze the synaptic potentials evoked by stationary sine-wave gratings. The model predicted the responses of cells to moving gratings with considerable accuracy. The degree of direction selectivity and the time course of the responses to moving gratings were both well matched by the model. The direction selectivity of the synaptic potentials was considerably smaller than that of the intracellularly recorded action potential, indicating that a nonlinear mechanism such as threshold enhances the direction selectivity of the cell's output over that of its synaptic inputs. At the input stage, however, the cells apparently sum their synaptic inputs in a highly linear fashion. A more constrained test of linearity of synaptic summation based on principal component analysis was applied to the responses of direction-selective cells to stationary gratings. The analysis confirms that the summation in these cells is highly linear. The principal component analysis is consistent with a model in which direction selectivity in cortical simple cells is generated by only two subunits, each with a different receptive-field position and response time course. The response time course for each of the two subunits is derived for four analyzed cells. Each derived subunit is linear in spatial summation, suggesting that the neurons that comprise each subunit are either geniculate X-cells or receive their primary synaptic input from X-cells. The amplitude of the response of each subunit is linearly related to the contrast of the stimulus. The subunits are nonlinear in the time domain, however: the response to a stationary stimulus whose contrast is modulated sinusoidally in time is nonsinusoidal. The principal component analysis does not exclude models of direction selectivity based on more than two subunits, but such higher-order models would have to include the constraint that the extra subunits form a smooth continuum of interpolation between the properties derived from the two subunit solution.

Animals↗
PubMed1993

Linearity of summation of synaptic potentials underlying direction selectivity in simple cells of the cat visual cortex.

Intracellular recordings from simple cells of the cat visual cortex were used to test linear models for the generation of selectivity for the direction of visual motion. Direction selectivity has been thought to arise in part from nonlinear processes, as suggested by previous experiments that were based on extracellular recordings of action potentials. In intracellular recordings, however, the fluctuations in membrane potential evoked by moving stimuli were accurately predicted by the linear summation of responses to stationary stimuli. Nonlinear mechanisms were not required.

Animals↗
PubMed1992

Visually evoked oscillations of membrane potential in cells of cat visual cortex.

In response to visual stimulation, cells of the cat visual cortex fire rhythmically at frequencies between 30 and 60 hertz. This rhythmic firing can be synchronized among cells in widespread areas of the visual cortex. The visual stimulus conditions under which this process occurs suggest that the synchronization may contribute to the integration of information across broadly displaced parts of the visual field. An intricate mechanism must control the regularity of firing and its synchronization. In vivo whole-cell patch recordings from cells in area 17 have now shown that robust oscillations of membrane potential underlie the regularity of firing seen extracellularly. In the cells studied, the characteristics of the oscillations of membrane potential suggest that such oscillations are produced by rhythmic activity in synaptic inputs. These rhythmic synaptic inputs form the most likely mechanism for the synchronization of activity in neighboring cortical cells.

Animals↗
PubMed1992

EPSP-IPSP interactions in cat visual cortex studied with in vivo whole-cell patch recording.

Postsynaptic inhibition can operate by two distinct mechanisms: (1) membrane hyperpolarization and (2) shunting of excitatory postsynaptic currents. The arithmetic operations--either addition or multiplication--that synapses are able to perform during neuronal computations are determined by which of these two inhibitory mechanisms predominates. Hyperpolarizing IPSPs interact linearly with EPSPs; their negative and positive synaptic currents sum to produce a net change in membrane potential (Eccles, 1961). Shunting synapses interact nonlinearly with EPSPs; the shunt-induced increase in membrane conductance directly reduces the amplitude of EPSPs by a constant multiplicative factor (Fatt and Katz, 1953; Blomfield, 1974). This property of shunting inhibition has provided the basis for models of synaptic interaction in which shunting inhibition acts as an AND-NOT gate for excitatory inputs (Torre and Poggio, 1978; Koch et al., 1983). Using an in vivo variant of the whole-cell patch technique (Blanton et al., 1989), we have examined the effect of visually evoked inhibition on the size of EPSPs in cortical simple cells and found that the predominant inhibitory mechanism is hyperpolarization. We conclude that these inhibitory synapses operate primarily in the linear mode.

Animals↗
PubMed1991

Nonlinearity of spatial summation in simple cells of areas 17 and 18 of cat visual cortex.

1. Nonlinearity of spatial summation in areas 17 and 18 of cat visual cortex was compared with the type of spatial nonlinearity that differentiates X and Y cells in the lateral geniculate nucleus (LGN) and retina. The comparisons were made to examine to what extent the information from X and Y cells may remain separated in higher visual centers. 2. Responses of simple cells in areas 17 and 18 were recorded while stationary, optimally oriented sinewave gratings were sinusoidally modulated within the receptive field of the cell. Both the spatial frequency and spatial phase of the stimulus were varied. 3. Y cells in the retina and LGN are defined by the presence of a specific form of spatial nonlinearity. When tested with contrast-modulated sinewave gratings of spatial frequencies about three-fold greater than the optimal, their responses are dominated by a frequency-doubled component. The amplitude of the frequency-doubled component is not dependent on the spatial phase of the stimulus. 4. Many simple cells in the cortex showed a form of spatial nonlinearity similar to the defining nonlinearity found in retinal and geniculate Y cells. A frequency-doubled response dominated at spatial frequencies more than threefold greater than the optimal spatial frequency. When this response was present, it was phase independent. 5. More than 50% of the simple cells in area 18 showed the Y-like spatial nonlinearity. Fewer than 10% of the simple cells in area 17 showed the Y-like spatial nonlinearity. 6. The virtual absence of Y-like nonlinearity in area 17 and its relative abundance in area 18 suggest that the functional separation between the parallel X and Y pathways remains distinct within areas 17 and 18 of cat visual cortex.

Animals↗