Acetylcholine (ACh) plays a permissive role in developmental plasticity of fibers from the lateral geniculate nucleus (LGN) to the primary visual cortex (V1). These fibers remain plastic and express long-term potentiation (LTP) in adult rodents, but it is not known if ACh modulates this form of plasticity in the mature V1. We show that, in anesthetized rats, theta burst stimulation (TBS) of the LGN using 5 or 40 theta cycles produced moderate (approximately 20%) and stronger (approximately 40%) potentiation, respectively, of field postsynaptic potentials recorded in the ipsilateral V1. Basal forebrain stimulation (100 Hz) 5 min after TBS enhanced LTP induced by both weak (5 theta cycles) and strong (40 theta cycles) induction protocols. Both effects were reduced by systemic administration of the muscarinic receptor antagonist scopolamine. Basal forebrain stimulation did not enhance LTP when applied 30 min after or 5 min prior to TBS, suggesting that ACh affects early LTP induction mechanisms. Application of the cholinergic agonist carbachol in V1 by means of reverse microdialysis mimicked the effect of basal forebrain stimulation. We conclude that heterosynaptic facilitation of V1 plasticity by ACh extends beyond early postnatal maturation periods and acts to convert weak potentiation into pronounced, long-lasting increases in synaptic strength.

Stretching the muscles of the buccal mass of Aplysia evoked proprioceptive reflexes. These consisted of a direct reflex in which the stretched muscle contracted and a crossed reflex in which the contralateral homolog of the stretched muscle contracted as well. Both reflexes were accompanied by corresponding changes in neural activity in the buccal nerves. The muscle contraction and efferent neural activity were abolished by blocking synaptic transmission in the buccal ganglia. Blocking neuromuscular transmission blocked the contractions but not the stretch-induced afferent neural activity. Proprioceptive responses were obtained from isolated buccal nerve-muscle preparations. Both tonic on- and on-off responses were observed. These responses persisted after blocking synaptic transmission at the muscle, indicating that they were due to afferent fibers rather than peripheral interneurons. Proprioceptive neurons with centrally located cell bodies were found. These included previously identified neurons B4 and B5 as well as small cells. Proprioceptive neurons responded to muscle stretch with peripherally initiated axonal spikes that conducted into the central nervous system (CNS) and preceded their somatic spikes. These responses persisted after blocking synaptic transmission in the CNS. Several motor neurons were found. When intracellularly stimulated, these evoked contractions of their target muscle even after blocking synaptic transmission in the CNS. The motor neurons responded synaptically to stretching the ipsilateral muscle. Some responded to stretching of the contralateral homologous muscle as well. The motor neurons differed in their axonal projections, with some projecting only ipsilaterally, others bilaterally. The majority of motor neurons were inhibited by muscle stretch due to inhibitory monosynaptic input from the proprioceptive cells B4 and B5. The stretch reflex occurred when the motor neurons fired due to postinhibitory rebound. The synaptic organization of the reflex was considered.

The results suggest that individual differences in silent speechreading and the McGurk effect are not related. This conclusion is supported by differential influences of high-resolution visual information on the 2 tasks and differences in the pattern of gaze.

Communication impairments pose a major threat to an individual's quality of life. However, the impact of visual impairments on communication is not well understood, despite the important role that vision plays in the perception of speech. Here we present 2 experiments examining the impact of discrete central scotomas on speech perception. In the first experiment, 4 patients with central vision loss due to unilateral macular holes identified utterances with conflicting auditory-visual information, while simultaneously having their eye movements recorded. Each eye was tested individually. Three participants showed similar speech perception with both the impaired eye and the unaffected eye. For 1 participant, speech perception was disrupted by the scotoma because the participant did not shift gaze to avoid obscuring the talker's mouth with the scotoma. In the second experiment, 12 undergraduate students with gaze-contingent artificial scotomas (10 visual degrees in diameter) identified sentences in background noise. These larger scotomas disrupted speech perception, but some participants overcame this by adopting a gaze strategy whereby they shifted gaze to prevent obscuring important regions of the face such as the mouth. Participants who did not spontaneously adopt an adaptive gaze strategy did not learn to do so over the course of 5 days; however, participants who began with adaptive gaze strategies became more consistent in their gaze location. These findings confirm that peripheral vision is sufficient for perception of most visual information in speech, and suggest that training in gaze strategy may be worthwhile for individuals with communication deficits due to visual impairments.

Japanese words can be distinguished by the length of phonemes, e.g., ‘‘chizu’’ (map) versus ‘‘chiizu’’ (cheese). Perceiving these length contrasts is therefore important for learning Japanese as a second language. The present study examined native English listeners’ perception of length contrasts at different speaking rates and in different contexts. Stimuli consisted of 20 Japanese word pairs that minimally contrasted in vowel length, and 10 synthesized nonwords. The nonwords were created by modifying the duration of the second vowel of the nonword ‘‘erete’’ along a continuum (from ‘‘erete’’ to ‘‘ereete’’). Stimuli were presented with or without a carrier sentence at three rates (fast, normal, slow). Rate was either fixed or randomized trial by trial. Sixteen native English and 16 native Japanese listeners participated in a single-stimulus, two-alternative forced-choice identification task. Results suggest that native Japanese listeners’ identification boundaries systematically shifted due to changes in speaking rate when the stimuli were in the context of a sentence with mixed rates of presentation. In contrast, native English listeners show a shift in the opposite direction, suggesting that they did not follow the variation in speaking rate. These results will be discussed from the viewpoint of training second-language phoneme perception. [Work supported by JSPS.]