Biomedical subjects

Kamal Sen

Publications and source records attributed to Kamal Sen.

5 recordsLinked to original sources

PubMed2006

Network architecture, receptive fields, and neuromodulation: computational and functional implications of cholinergic modulation in primary auditory cortex.

Two fundamental issues in auditory cortical processing are the relative importance of thalamocortical versus intracortical circuits in shaping response properties in primary auditory cortex (ACx), and how the effects of neuromodulators on these circuits affect dynamic changes in network and receptive field properties that enhance signal processing and adaptive behavior. To investigate these issues, we developed a computational model of layers III and IV (LIII/IV) of AI, constrained by anatomical and physiological data. We focus on how the local and global cortical architecture shape receptive fields (RFs) of cortical cells and on how different well-established cholinergic effects on the cortical network reshape frequency-tuning properties of cells in ACx. We identify key thalamocortical and intracortical circuits that strongly affect tuning curves of model cortical neurons and are also sensitive to cholinergic modulation. We then study how differential cholinergic modulation of network parameters change the tuning properties of our model cells and propose two different mechanisms: one intracortical (involving muscarinic receptors) and one thalamocortical (involving nicotinic receptors), which may be involved in rapid plasticity in ACx, as recently reported in a study by Fritz and coworkers.

Acoustic Stimulation↗

PubMed2006

Distinct time scales in cortical discrimination of natural sounds in songbirds.

Understanding how single cortical neurons discriminate between sensory stimuli is fundamental to providing a link between cortical neural responses and perception. The discrimination of sensory stimuli by cortical neurons has been intensively investigated in the visual and somatosensory systems. However, relatively little is known about discrimination of sounds by auditory cortical neurons. Auditory cortex plays a particularly important role in the discrimination of complex sounds, e.g., vocal communication sounds. The rich dynamic structure of such complex sounds on multiple time scales motivates two questions regarding cortical discrimination. How does discrimination depend on the temporal resolution of the cortical response? How does discrimination accuracy evolve over time? Here we investigate these questions in field L, the analogue of primary auditory cortex in zebra finches, analyzing temporal resolution and temporal integration in the discrimination of conspecific songs (songs of the bird's own species) for both anesthetized and awake subjects. We demonstrate the existence of distinct time scales for temporal resolution and temporal integration and explain how they arise from cortical neural responses to complex dynamic sounds.

Animals↗

PubMed2005

Increasing Ca2+ transients by broadening postsynaptic action potentials enhances timing-dependent synaptic depression.

Repeated induction of pre- and postsynaptic action potentials (APs) at a fixed time difference leads to long-term potentiation (LTP) or long-term depression (LTD) of the synapse, depending on the temporal order of pre- and postsynaptic activity. This phenomenon of spike-timing-dependent plasticity (STDP) is believed to arise by nonlinear processes that lead to larger calcium transients (and thus LTP) when presynaptic APs precede postsynaptic APs and smaller calcium transients (and thus LTD) when postsynaptic APs precede presynaptic APs. In contrast to predictions from such calcium-peak-detector models, we show that constitutively or artificially broadened APs in layer II/III pyramidal cells of entorhinal cortex (EC) lead to an increase in the dendritic calcium transient and shift the balance of STDP toward LTD. STDP in entorhinal pyramidal cells is NMDA-receptor-dependent and modulated by the Ca(V)1Ca(2+) channel-blocker nifedipine. Results are consistent with an elaboration of the calcium-peak-detector model in which downstream signals from voltage-dependent Ca(2+) channels suppress LTP relative to LTD. Our results suggest that modulation of AP width is a potent way to adjust the rules of synaptic plasticity in the EC.

Action Potentials↗

PubMed2005

Delayed inhibition in cortical receptive fields and the discrimination of complex stimuli.

Although auditory cortex is thought to play an important role in processing complex natural sounds such as speech and animal vocalizations, the specific functional roles of cortical receptive fields (RFs) remain unclear. Here, we study the relationship between a behaviorally important function: the discrimination of natural sounds and the structure of cortical RFs. We examine this problem in the model system of songbirds, using a computational approach. First, we constructed model neurons based on the spectral temporal RF (STRF), a widely used description of auditory cortical RFs. We focused on delayed inhibitory STRFs, a class of STRFs experimentally observed in primary auditory cortex (ACx) and its analog in songbirds (field L), which consist of an excitatory subregion and a delayed inhibitory subregion cotuned to a characteristic frequency. We quantified the discrimination of birdsongs by model neurons, examining both the dynamics and temporal resolution of discrimination, using a recently proposed spike distance metric (SDM). We found that single model neurons with delayed inhibitory STRFs can discriminate accurately between songs. Discrimination improves dramatically when the temporal structure of the neural response at fine timescales is considered. When we compared discrimination by model neurons with and without the inhibitory subregion, we found that the presence of the inhibitory subregion can improve discrimination. Finally, we modeled a cortical microcircuit with delayed synaptic inhibition, a candidate mechanism underlying delayed inhibitory STRFs, and showed that blocking inhibition in this model circuit degrades discrimination.

Acoustic Stimulation↗

PubMed2005

Spatial unmasking of birdsong in human listeners: energetic and informational factors.

Spatial unmasking describes the improvement in the detection or identification of a target sound afforded by separating it spatially from simultaneous masking sounds. This effect has been studied extensively for speech intelligibility in the presence of interfering sounds. In the current study, listeners identified zebra finch song, which shares many acoustic properties with speech but lacks semantic and linguistic content. Three maskers with the same long-term spectral content but different short-term statistics were used: (1) chorus (combinations of unfamiliar zebra finch songs), (2) song-shaped noise (broadband noise with the average spectrum of chorus), and (3) chorus-modulated noise (song-shaped noise multiplied by the broadband envelope from a chorus masker). The amount of masking and spatial unmasking depended on the masker and there was evidence of release from both energetic and informational masking. Spatial unmasking was greatest for the statistically similar chorus masker. For the two noise maskers, there was less spatial unmasking and it was wholly accounted for by the relative target and masker levels at the acoustically better ear. The results share many features with analogous results using speech targets, suggesting that spatial separation aids in the segregation of complex natural sounds through mechanisms that are not specific to speech.

Acoustic Stimulation↗