Biomedical subjects

Michael P Kilgard

Publications and source records attributed to Michael P Kilgard.

8 recordsLinked to original sources

PubMed2006

Cortical plasticity and rehabilitation.

The brain is constantly adapting to environmental and endogenous changes (including injury) that occur at every stage of life. The mechanisms that regulate neural plasticity have been refined over millions of years. Motivation and sensory experience directly shape the rewiring that makes learning and neurological recovery possible. Guiding neural reorganization in a manner that facilitates recovery of function is a primary goal of neurological rehabilitation. As the rules that govern neural plasticity become better understood, it will be possible to manipulate the sensory and motor experience of patients to induce specific forms of plasticity. This review summarizes our current knowledge regarding factors that regulate cortical plasticity, illustrates specific forms of reorganization induced by control of each factor, and suggests how to exploit these factors for clinical benefit.

Brain Injuries↗

PubMed2005

Cortical map reorganization without cholinergic modulation.

Acetylcholine has been shown to modulate many forms of cortical plasticity. New evidence indicates that reorganization of adult primary auditory cortex is still possible after removal of cholinergic inputs. This finding suggests that acetylcholine may act less as a gate and more as a gain control on cortical plasticity.

Acetylcholine↗

PubMed2005

Environmental enrichment increases paired-pulse depression in rat auditory cortex.

Temporal features are important for the identification of natural sounds. Earlier studies have shown that cortical processing of temporal information can be altered by long-term experience with modulated sounds. In a previous study, we observed that environmental enrichment dramatically increased the response of cortical neurons to single tone and noise burst stimuli in both awake and anesthetized rats. Here, we evaluate how enrichment influences temporal information processing in the auditory cortex. We recorded responses to repeated tones and noise bursts in awake rats using epidural evoked potentials and in anesthetized rats using microelectrodes. Enrichment increased the response of cortical neurons to stimuli presented at slow rates and decreased the response to stimuli presented at fast rates relative to controls. Our observation that enrichment substantially increased response strength and forward masking is consistent with earlier reports that long-term potentiation of cortical synapses is associated with increased paired-pulse depression. Enrichment also increased response synchronization at slow rates and decreased synchronization at fast rates. Paired-pulse depression increased within days of environmental enrichment and was restored to normal levels after return to standard housing conditions. These results are relevant to several clinical disorders characterized by abnormal gating of sensory information, including autism, schizophrenia, and dyslexia.

Acoustic Stimulation↗

PubMed2005

Asynchronous inputs alter excitability, spike timing, and topography in primary auditory cortex.

Correlation-based synaptic plasticity provides a potential cellular mechanism for learning and memory. Studies in the visual and somatosensory systems have shown that behavioral and surgical manipulation of sensory inputs leads to changes in cortical organization that are consistent with the operation of these learning rules. In this study, we examine how the organization of primary auditory cortex (A1) is altered by tones designed to decrease the average input correlation across the frequency map. After one month of separately pairing nucleus basalis stimulation with 2 and 14 kHz tones, a greater proportion of A1 neurons responded to frequencies below 2 kHz and above 14 kHz. Despite the expanded representation of these tones, cortical excitability was specifically reduced in the high and low frequency regions of A1, as evidenced by increased neural thresholds and decreased response strength. In contrast, in the frequency region between the two paired tones, driven rates were unaffected and spontaneous firing rate was increased. Neural response latencies were increased across the frequency map when nucleus basalis stimulation was associated with asynchronous activation of the high and low frequency regions of A1. This set of changes did not occur when pulsed noise bursts were paired with nucleus basalis stimulation. These results are consistent with earlier observations that sensory input statistics can shape cortical map organization and spike timing.

Acoustic Stimulation↗

PubMed2004

Background sounds contribute to spectrotemporal plasticity in primary auditory cortex.

The mammalian auditory system evolved to extract meaningful information from complex acoustic environments. Spectrotemporal selectivity of auditory neurons provides a potential mechanism to represent natural sounds. Experience-dependent plasticity mechanisms can remodel the spectrotemporal selectivity of neurons in primary auditory cortex (A1). Electrical stimulation of the cholinergic nucleus basalis (NB) enables plasticity in A1 that parallels natural learning and is specific to acoustic features associated with NB activity. In this study, we used NB stimulation to explore how cortical networks reorganize after experience with frequency-modulated (FM) sweeps, and how background stimuli contribute to spectrotemporal plasticity in rat auditory cortex. Pairing an 8-4 kHz FM sweep with NB stimulation 300 times per day for 20 days decreased tone thresholds, frequency selectivity, and response latency of A1 neurons in the region of the tonotopic map activated by the sound. In an attempt to modify neuronal response properties across all of A1 the same NB activation was paired in a second group of rats with five downward FM sweeps, each spanning a different octave. No changes in FM selectivity or receptive field (RF) structure were observed when the neural activation was distributed across the cortical surface. However, the addition of unpaired background sweeps of different rates or direction was sufficient to alter RF characteristics across the tonotopic map in a third group of rats. These results extend earlier observations that cortical neurons can develop stimulus specific plasticity and indicate that background conditions can strongly influence cortical plasticity.

Acoustic Stimulation↗

PubMed2004

Environmental enrichment improves response strength, threshold, selectivity, and latency of auditory cortex neurons.

Over the last 50 yr, environmental enrichment has been shown to generate more than a dozen changes in brain anatomy. The consequences of these physical changes on information processing have not been well studied. In this study, rats were housed in enriched or standard conditions either prior to or after reaching sexual maturity. Evoked potentials from awake rats and extracellular recordings from anesthetized rats were used to document responses of auditory cortex neurons. This report details several significant, new findings about the influence of housing conditions on the responses of rat auditory cortex neurons. First, enrichment dramatically increases the strength of auditory cortex responses. Tone-evoked potentials of enriched rats, for example, were more than twice the amplitude of rats raised in standard laboratory conditions. Second, cortical responses of both young and adult animals benefit from exposure to an enriched environment and are degraded by exposure to an impoverished environment. Third, housing condition resulted in rapid remodeling of cortical responses in <2 wk. Fourth, recordings made under anesthesia indicate that enrichment increases the number of neurons activated by any sound. This finding shows that the evoked potential plasticity documented in awake rats was not due to differences in behavioral state. Finally, enrichment made primary auditory cortex (A1) neurons more sensitive to quiet sounds, more selective for tone frequency, and altered their response latencies. These experiments provide the first evidence of physiologic changes in auditory cortex processing resulting from generalized environmental enrichment.

Acoustic Stimulation↗

PubMed2002

Order-sensitive plasticity in adult primary auditory cortex.

The neural response to a stimulus presented as part of a rapid sequence is often quite different from the response to the same stimulus presented in isolation. In primary auditory cortex (A1), although the most common effect of preceding stimuli is inhibitory, most neurons can also exhibit response facilitation if the appropriate spectral and temporal separation of sequence elements is presented. In this study, we investigated whether A1 neurons in adult animals can develop context-dependent facilitation to a novel acoustic sequence. After repeatedly pairing electrical stimulation of the basal forebrain with a three-element sequence (high frequency tone--low frequency tone-- noise burst), 25% of A1 neurons exhibited facilitation to the low tone when preceded by the high tone, compared with only 5% in controls. In contrast, there was no increase in the percent of sites that showed facilitation for the reversed tone order (low preceding high). Nearly 60% of sites exhibited a facilitated response to the noise burst when preceded by the two tones. Although facilitation was greatest in response to the paired sequence, facilitation also generalized to related sequences that were either temporally distorted or missing one of the tones. Pairing basal forebrain stimulation with the acoustic sequence also caused a decrease in the time to peak response and an increase in population discharge synchrony, which was not seen after pairing simple tones, tone trains, or broadband stimuli. These results indicate that context-dependent facilitation and response synchronization can be substantially altered in an experience-dependent fashion and provide a potential mechanism for learning spectrotemporal patterns.

Animals↗

PubMed2002

Cortical network reorganization guided by sensory input features.

Sensory experience alters the functional organization of cortical networks. Previous studies using behavioral training motivated by aversive or rewarding stimuli have demonstrated that cortical plasticity is specific to salient inputs in the sensory environment. Sensory experience associated with electrical activation of the basal forebrain (BasF) generates similar input specific plasticity. By directly engaging plasticity mechanisms and avoiding extensive behavioral training, BasF stimulation makes it possible to efficiently explore how specific sensory features contribute to cortical plasticity. This review summarizes our observations that cortical networks employ a variety of strategies to improve the representation of the sensory environment. Different combinations of receptive-field, temporal, and spectrotemporal plasticity were generated in primary auditory cortex neurons depending on the pitch, modulation rate, and order of sounds paired with BasF stimulation. Simple tones led to map expansion, while modulated tones altered the maximum cortical following rate. Exposure to complex acoustic sequences led to the development of combination-sensitive responses. This remodeling of cortical response characteristics may reflect changes in intrinsic cellular mechanisms, synaptic efficacy, and local neuronal connectivity. The intricate relationship between the pattern of sensory activation and cortical plasticity suggests that network-level rules alter the functional organization of the cortex to generate the most behaviorally useful representation of the sensory environment.

Acetylcholine↗