Speech comprehension depends on the integrity of both the spectral content and temporal envelope of the speech signal. Although neural processing underlying spectral analysis has been intensively studied, less is known about the processing of temporal information. Most of speech information conveyed by the temporal envelope is confined to frequencies below 16 Hz, frequencies that roughly match spontaneous and evoked modulation rates of primary auditory cortex neurons. To test the importance of cortical modulation rates for speech processing, we manipulated the frequency of the temporal envelope of speech sentences and tested the effect on both speech comprehension and cortical activity. Magnetoencephalographic signals from the auditory cortices of human subjects were recorded while they were performing a speech comprehension task. The test sentences used in this task were compressed in time. Speech comprehension was degraded when sentence stimuli were presented in more rapid (more compressed) forms. We found that the average comprehension level, at each compression, correlated with (i) the similarity between the frequencies of the temporal envelopes of the stimulus and the subject's cortical activity (''stimulus-cortex frequency-matching'') and (ii) the phase-locking (PL) between the two temporal envelopes (''stimulus-cortex PL''). Of these two correlates, PL was significantly more indicative for single-trial success. Our results suggest that the match between the speech rate and the a priori modulation capacities of the auditory cortex is a prerequisite for comprehension. However, this is not sufficient: stimulus-cortex PL should be achieved during actual sentence presentation.human ͉ MEG ͉ time compression ͉ accelerated speech ͉ phase-locking C omprehension of speech depends on the integrity of its temporal envelope, that is, on the temporal variations of spectral energy. The temporal envelope contains information that is essential for the identification of phonemes, syllables, words, and sentences (1). Envelope frequencies of normal speech are usually below 8 Hz (ref. 2; see Figs. 1 and 2). The critical frequency band of the temporal envelope for normal speech comprehension is between 4 and 16 Hz (3, 4); envelope details above 16 Hz have only a small [although significant (5)] effect on comprehension. Across this low-frequency modulation range, comprehension does not usually depend on the exact frequencies of the temporal envelopes of incoming speech, because the temporal envelope of normal speech can be compressed in time down to 0.5 of its original duration before comprehension is significantly affected (6, 7). Thus, normal brain mechanisms responsible for speech perception can adapt to different input rates within this range (see refs. 8-10). This online adaptation is crucial for speech perception, because speech rates vary between different speakers and change according to the emotional state of the speaker.Poor readers, many of whom have poor successive-signal auditory (11-17) and visual (18) processing,...

The magnocellular theory is a prominent, albeit controversial view asserting that many reading disabled (RD) individuals suffer from a specific impairment within the visual magnocellular pathway. In order to assess the validity of this theory we tested its two basic predictions. The first is that a subpopulation of RD subjects will show impaired performance across a broad range of psychophysical tasks relying on magnocellular functions. The second is that this subpopulation will not be consistently impaired across tasks that do not rely on magnocellular functions. We defined a behavioural criterion for magnocellular function, which incorporates performance in flicker detection, detection of drifting gratings (at low spatial frequencies), speed discrimination and detection of coherent dot motion. We found that some RD subjects (six out of 30) had impaired magnocellular function. Nevertheless, these RD subjects were also consistently impaired on a broad range of other perceptual tasks. The performance of the other subgroup of RD subjects on magnocellular tasks did not differ from that of controls. However, they did show impaired performance in both visual and auditory non-magnocellular tasks requiring fine frequency discriminations. The stimuli used in these tasks were neither modulated in time nor briefly presented. We conclude that some RD subjects have generally impaired perceptual skills. Many RD subjects have more specific perceptual deficits; however, the "magnocellular" level of description did not capture the nature of the perceptual difficulties in any of the RD individuals assessed by us.