Itzel Orduña

Basal forebrain stimulation changes cortical sensitivities to complex sound

Experience affects how brains respond to sound. Here, we examined how the sensitivity and selectivity of auditory cortical neuronal responses were affected in adult rats by the repeated presentation of a complex sound that was paired with basal forebrain stimulation. The auditory cortical region that was responsive to complex sound was 2–5 five times greater in area in paired-stimulation rats than in naive rats. Magnitudes of neuronal responses evoked by complex sounds were also greatly increased by associative pairing, as were the percentages of neurons that responded selectively to the specific spectrotemporal features that were paired with stimulation. These findings demonstrate that feature selectivity within the auditory cortex can be flexibly altered in adult mammals through appropriate intensive training.

Spectrotemporal sensitivities in rat auditory cortical neurons

Studies in several mammalian species have demonstrated that auditory cortical neurons respond strongly to single frequency-modulated (FM) sweeps, and that most responses are selective for sweep direction and/or rate. In the present study, we used extracellular recordings to examine how neurons in the auditory cortices of anesthetized rats respond to continuous, periodic trains of FM sweeps (described previously by deCharms et al., Science 280 (1998) pp. 1439-1444, as moving auditory gratings). Consistent with previous observations in owl monkeys, we found that the majority of cortical neurons responded selectively to trains of either up-sweeps or down-sweeps; selectivity for down-sweeps was most common. Periodic responses were typically evoked only by sweep trains with repetition rates less than 12 sweeps per second. Directional differences in responses were dependent on repetition rate. Our results support the proposal that a combination of both spectral and temporal acoustic features determines the responses of auditory cortical neurons to sound, and add to the growing body of evidence indicating that the traditional view of the auditory cortex as a frequency analyzer is not sufficient to explain how the mammalian brain represents complex sounds.

Evoked-potential changes following discrimination learning involving complex sounds

OBJECTIVE Perceptual sensitivities are malleable via learning, even in adults. We trained adults to discriminate complex sounds (periodic, frequency-modulated sweep trains) using two different training procedures, and used psychoacoustic tests and evoked potential measures (the N1-P2 complex) to assess changes in both perceptual and neural sensitivities. METHODS Training took place either on a single day, or daily across eight days, and involved discrimination of pairs of stimuli using a single-interval, forced-choice task. In some participants, training started with dissimilar pairs that became progressively more similar across sessions, whereas in others training was constant, involving only one, highly similar, stimulus pair. RESULTS Participants were better able to discriminate the complex sounds after training, particularly after progressive training, and the evoked potentials elicited by some of the sounds increased in amplitude following training. Significant amplitude changes were restricted to the P2 peak. CONCLUSIONS Our findings indicate that changes in perceptual sensitivities parallel enhanced neural processing. SIGNIFICANCE These results are consistent with the proposal that changes in perceptual abilities arise from the brains capacity to adaptively modify cortical representations of sensory stimuli, and that different training regimens can lead to differences in cortical sensitivities, even after relatively short periods of training.

Cortical responses in rats predict perceptual sensitivities to complex sounds

The common assumption that perceptual sensitivities are related to neural representations of sensory stimuli has seldom been directly demonstrated. The authors analyzed the similarity of spike trains evoked by complex sounds in the rat auditory cortex and related cortical responses to performance in an auditory task. Rats initially learned to identify 2 highly different periodic, frequency-modulated sounds and then were tested with increasingly similar sounds. Rats correctly classified most novel sounds; their accuracy was negatively correlated with acoustic similarity. Rats discriminated novel sounds with slower modulation more accurately than sounds with faster modulation. This asymmetry was consistent with similarities in cortical representations of the sounds, demonstrating that perceptual sensitivities to complex sounds can be predicted from the cortical responses they evoke.

Selective auditory attention to features of complex sounds: A comparative approach

When listeners are trained to respond based on one spectrotemporal component of a complex sound, enhanced processing of the behaviorally relevant feature provides an objective correlate of selective attention [I. J. Hirsh and C. S. Watson, Annu. Rev. Psychol. 47, 461–484 (1996)]. To study this issue in a nonhuman species, rats were trained to classify multidimensional acoustic stimuli based on the rate, direction, and range of frequency modulation. Rats successfully learned to classify complex sounds along the dimensions of rate and direction of frequency modulation, but not based on the range of frequency modulation. Rats classified stimuli most accurately when the relevant dimension was rate of frequency modulation. The relative ease with which rats learn to classify complex sounds along a particular dimension can be predicted based on how auditory cortical neurons in rats respond to such sounds. These findings provide new insights into how neural processing may constrain selective auditory attention to...

Plasticity of spectrotemporal sensitivities in auditory cortex

Response characteristics of auditory cortex can be altered by repeatedly pairing sounds with basal forebrain stimulation [M. P. Kilgard and M. M. Merzenich, Science 279, 1714–1718 (1998)]. Although many neurons in auditory cortex respond most strongly to time‐varying sounds, most studies of stimulation‐induced plasticity have focused on changes in responses to tone pips. We examined stimulation‐induced changes in neuronal sensitivities to frequency‐modulated sweep trains (bandwidth=2–16 kHz, duration=1 s, sweep rates=4–24 octaves/s, repetition rates=2–24 sweeps/s). Adult rats received electrical stimulation of basal forebrain paired with 1–10 varieties of sweep trains, 300–500 times per day, for 9–16 days. Some sounds were presented in combination with bandlimited Gaussian noise. After stimulation, neuronal responses were recorded from 20–80 sites in the auditory cortex of each rat. The spectrotemporal sensitivities of auditory cortical neurons were dramatically altered in stimulated rats. Changes in resp...

Experience‐dependent cortical processing of complex sounds by rats

Auditory cortex is thought to play a critical role in the processing of species‐specific vocalizations and other acoustically complex sounds. Although evolutionary processes strongly constrain cortical sensitivities to sound, cortical processing is not fixed by biology, but rather is shaped by the auditory experiences of each individual. Auditory cortical neurons in adult rats respond selectively to spectrotemporal features of complex sounds. These selective responses are predictive of rats’ behaviorally measured perceptual sensitivities. With extensive training, the abilities of rats to discriminate frequency‐modulated sounds improve. Recordings from cortical neurons in trained rats show increased sensitivities to features of the sounds used in training. These results demonstrate that discrimination training with biologically irrelevant complex sounds can change how cortical neurons process those sounds. Changes in cortical processing of complex sounds can also be induced by controlling activity in neuro...

Cortical responses in rats to periodic frequency‐modulated sounds

Spectral variations over time are a common characteristic of naturally occurring sounds. They constitute a prevalent feature in communication signals in several species. Studies of mammals have shown that auditory cortex neurons respond to single frequency‐modulated (FM) sweeps and that most responses are selective for sweep direction and/or rate. Researchers have also used trains of FM sweeps to estimate spectrotemporal receptive fields in auditory cortex. In the present study, microelectrode recordings were used to explore how the auditory cortex responds to trains of FM sweeps in anesthetized rats. Maps of 20–60 penetrations were made for each subject. Sweep frequencies ranged from 1–16 kHz with FM rates ranging from 4–24 octaves/s and repetition rates from 2–24 sweeps/s. Both down‐sweeps and up‐sweeps were presented. Neuronal responses were analyzed in terms of onset, offset, oscillatory and directionally selective properties. Several types of responses predominated. Most units responded to sound onse...