Biomedical subjects

K Alho

Publications and source records attributed to K Alho.

At least 73 records · Page 4Linked to original sources

PubMed1994

Lesions of frontal cortex diminish the auditory mismatch negativity.

Event-related brain potentials to non-attended auditory stimuli were recorded from patients with dorsolateral prefrontal cortex (DPFCx) lesions and from age-matched control subjects as they performed a visual reaction time task. Auditory stimuli consisted of monaural sequences of repetitive standard tones (1000 Hz) and occasional deviant tones of a higher frequency (1300 Hz). In comparison with control subjects, DPFCx patients showed enhanced P1 amplitudes (mean peak latency 50 msec), consistent with reduced frontally mediated gating of sensory input to the auditory cortex. The mismatch negativity (MMN) elicited by deviant tones was reduced in DPFCx patients over a broad latency range (130-210 msec), especially over the lesioned hemisphere and for tones delivered to the ear ipsilateral to the lesion. The results suggest that DPFCx and DPFCx-temporal projections play a critical role in involuntary orienting to physical changes in sequences of non-attended auditory stimuli.

Acoustic Stimulation↗

PubMed1994

Magnetoencephalography in studies of human cognitive brain function.

Magnetoencephalography provides a new dimension to the functional imaging of the brain. The cerebral magnetic fields recorded noninvasively enable the accurate determination of locations of cerebral activity with an uncompromized time resolution. The first whole-scalp sensor arrays have just recently come into operation, and significant advances are to be expected in both neurophysiological and cognitive studies, as well as in clinical practice. However, although the accuracy of locating isolated sources of brain activity has improved, identification of multiple simultaneous sources can still be a problem. Therefore, attempts are being made to combine magnetoencephalography with other brain-imaging methods to improve spatial localization of multiple sources and, simultaneously, to achieve a more complete characterization of different aspects of brain activity during cognitive processing. Owing to its good time resolution and considerably better spatial accuracy than that provided by EEG, magnetoencephalography holds great promise as a tool for revealing information-processing sequences of the human brain.

Brain↗

PubMed1994

Strongly focused attention and auditory event-related potentials.

Effects of selective attention on the auditory event-related potential were studied by delivering two tones in a rapid, randomized sequence. One of the tones was designated as relevant and the other as irrelevant, the subject's task being to discriminate occasional softer tones within the relevant tones. Relevant and irrelevant tones differed from each other either in location (left vs. right ear) or pitch (300 vs. 6000 Hz). In both conditions, relevant tones elicited a larger N1 deflection (peak latency about 120 ms) than did irrelevant tones. In contrast to the exogenous N1 elicited by unattended tones, the scalp distribution of this attentional "N1 effect" did not depend on the stimulated ear and, further, its amplitude was not affected by the frequency of attended tones. These results suggest that the present attention effect was caused, at least mainly, by an endogenous processing negativity rather than by an enhancement of exogenous N1 components.

Adult↗

PubMed1994

Stages of auditory feature conjunction: an event-related brain potential study.

Auditory event-related brain potentials (ERPs) were recorded to tones of different frequencies and locations in a dichotic selective attention task in which Ss responded to occasional deviant tones of a prespecified location and frequency. Attention effects were isolated as negative difference (Nd) waves by subtracting ERPs to tones with no attended features from ERPs to the same tones when they shared target frequency, location, or both cues. The N1/P90 (latency 80-100 ms), originating in a tonotopically organized generator, was enhanced for all tones in the attended ear. Nd waves, beginning at 80 ms and lasting up to 700 ms, were seen to tones with either attended feature. Nd waves to frequency and location features had different scalp distributions consistent with generation in different cortical fields. Conjunction-specific Nds began 30-50 ms after Nds to individual features. The relative timing suggests that feature conjunction began before the analysis of individual features was complete.

Acoustic Stimulation↗

PubMed1994

Processing of auditory stimuli during auditory and visual attention as revealed by event-related potentials.

Auditory event-related brain potentials (ERPs) were recorded during auditory and visual selective attention tasks. Auditory stimuli consisted of frequent standard tones (1000 Hz) and infrequent deviant tones (1050 Hz and 1300 Hz) delivered randomly to the left and right ears. Visual stimuli were vertical line gratings randomly presented on a video monitor at mean intervals of 6 s. During auditory attention, the subject attended to the stimuli in a designated ear and responded to the 1300-Hz deviants occurring among the attended tones. During visual attention, the subject responded to the occasional visual stimuli. ERPs for tones delivered to the attended ear were negatively displaced relative to ERPs elicited by tones delivered to the unattended ear and to ERPs elicited by auditory stimuli during visual attention. This attention effect consisted of negative difference waves with early and late components. Mismatch negativities (MMNs) were elicited by 1300-Hz and 1050-Hz deviants irrespective of whether they occurred among attended or unattended tones. MMN amplitudes were unaffected by attention, supporting the proposal that the MMN is generated by an automatic cerebral discrimination process.

Adult↗

PubMed1993

Interaction between representations of different features of auditory sensory memory.

The neurophysiological basis of sensory memory was studied by measuring the magnetic counterpart (MMNm) of the mismatch negativity (MMN) with a whole-head 122-channel magnetometer. The MMNm is a response to a difference in the presented stimulus and a neuronal memory trace formed by repeated standard stimuli. This trace must contain information about the feature differing in the deviant. Keeping one feature (frequency) constant, we studied how other stimulus features affect the strength of the MMNm. The MMNm to a frequency change was weaker when the other features varied than when they were constant. This suggests that the MMNm to a frequency change is not independent of other stimulus features.

Acoustic Stimulation↗

PubMed1993

Selective attention enhances the auditory 40-Hz transient response in humans.

Studies of human auditory and somatosensory modalities have shown that there is an oscillatory response in the gamma-band (at about 40 Hz) frequency which is elicited by either steady state or transient stimulation. The auditory 40-Hz response is generated at least partially in the auditory cortex as a result of thalamocortical interaction and may serve perceptual integration and conscious perception. A connection to selective attention has been implied in human and animal studies, although the evidence is inconclusive. Moreover, fundamental differences between the human and animal 40-Hz responses prohibit generalization. Furthermore, most experiments have used steady-state stimulation during which the brain does not regain its resting state between stimuli as it does when transient stimulation is used. Here we study the effect of selective attention on the auditory gamma-band (40-Hz) transient response using subjects listening to tone pips presented in one ear while ignoring a concurrent sequence of tone pips in the other ear. The 40-Hz response was larger when subjects paid attention to stimuli rather than ignored them. This attention effect was most pronounced over the frontal and central scalp areas. Our results demonstrate a physiological correlate of selective attention in the 40-Hz transient response in humans.

Acoustic Stimulation↗

PubMed1993

Auditory processing in visual brain areas of the early blind: evidence from event-related potentials.

Auditory event-related potentials (ERPs) were recorded in early blind subjects and sighted controls when they attended to stimuli delivered to a designated ear under dichotic conditions. The scalp distribution of the processing negativity (PN), the endogenous negativity elicited by attended stimuli, was in the blind posterior to that in the sighted. This suggests that posterior brain areas normally involved in vision participate in auditory selective attention in the early blind. Furthermore, occasional higher-frequency tones in the to-be-ignored ear elicited a negativity (presumably the mismatch negativity; MMN) that had a posterior scalp distribution in the blind as compared to controls. This suggests that the posterior brain areas of the blind also participate in processing of auditory stimulus changes occurring outside the focus of attention.

Acoustic Stimulation↗

PubMed1993

Mismatch negativity to slight pitch changes outside strong attentional focus.

The mismatch negativity (MMN) component of the auditory event-related potential (ERP) is elicited by infrequent, physically deviant stimuli in a sequence of frequent homogeneous stimuli ("standards"). It has been suggested that the MMN is generated by an automatic (attention-independent) neural mismatch process between the sensory input and a memory trace encoding the physical features of the standard stimulus. The MMN independence of attention was addressed in the present study. Standard stimuli and two types of deviant stimuli, differing from standards in frequency either "widely" (5%) or "slightly" (3%) were dichotically presented in random order at a very rapid rate. The subject attended either to left- or right-ear stimuli, counting the number of "slight deviants" in that ear. A reading condition with the same stimuli was also included. Even in the present attend conditions with very strong attentional focus, the MMN was elicited by the slight deviants in the unattended input stream. Furthermore, its amplitude was similar to that of the MMN elicited by the equivalent deviant stimuli during reading. The results suggest that auditory frequency is fully analyzed even in the absence of attention.

Adult↗

PubMed1993

Memory-related processing of complex sound patterns in human auditory cortex: a MEG study.

Responses of the human brain to a complex sound pattern were recorded with a 24 channel magnetometer. The sound pattern consisted of 9 successive 50 ms segments, each with a different frequency. An infrequent change in the frequency of one of the segments elicited a magnetic mismatch response (MMNm) which peaked at about 200 ms after the deviant segment onset and resembled the electrical mismatch negativity (MMN). The equivalent current dipole which best explained the MMNm was located in the supratemporal auditory cortex, suggesting that a memory trace for the sound pattern was stored in that region.

Adult↗

PubMed1993

Attention and mismatch negativity.

The mismatch negativity (MMN) component of the auditory event-related potential (ERP) is elicited by infrequent, physically deviant stimuli in a sequence of frequent homogeneous stimuli (standards). It has been suggested that the MMN is generated by an automatic (attention-independent) neural mismatch process with a memory trace that encodes the physical features of the standard stimulus. The proposed MMN independence of attention was addressed in the present study. Standard stimuli and two types of deviant stimuli, differing from standards either in frequency or intensity, were dichotically presented in random order and at a rapid rate. The subject attended either to left- or right-ear stimuli, counting the number of a designated type of deviants in that ear. In the present conditions of very strongly focused attention, the MMN was elicited even by frequency change in the ignored input stream, and its amplitude was very similar to that of the MMN elicited by equivalent deviant stimuli (targets) in the attended input stream. In contrast, the MMN to intensity deviation was clearly attenuated in the absence of attention. This effect is, however, probably due to the attention effect on the MMN generator itself rather than the antecedent sensory-analysis and -storing functions.

Acoustic Stimulation↗

PubMed1993

Tonotopic auditory cortex and the magnetoencephalographic (MEG) equivalent of the mismatch negativity.

Two tone stimuli, one frequent (standard) and the other infrequent (a slightly higher, deviant tone), were presented in random order and at short intervals to subjects reading texts they had selected. In different blocks, standards were either 250, 1,000, or 4,000 Hz, with the deviants always being 10% higher in frequency than the standards of the same blocks. Magnetic responses elicited by the standard and deviant tones included N1m, the magnetoencephalographic equivalent of the electrical N1 (its supratemporal component). In addition, deviant stimuli elicited MMNm, the magnetic equivalent of the electrical mismatch negativity, MMN. The equivalent dipole sources of the two responses were located in supratemporal auditory cortex, with the MMNm source being anterior to that of N1m. The dipole orientations of both sources in teh sagittal plane depended on stimulus frequency, suggesting that the responses are generated by tonotopically organized neuronal populations. The tonotopy reflected by the frequency dependence of the MMNm source might be that of the neural trace system underlying frequency representation of auditory stimuli in sensory memory.

Acoustic Stimulation↗

PubMed1993

Interstimulus interval and the selective-attention effect on auditory ERPs: "N1 enhancement" versus processing negativity.

The attention effect on the auditory event-related potential (ERP) in dichotic conditions was studied as a function of the interstimulus interval (ISI). Subjects attended to stimuli delivered to a designated ear and responded to infrequent pitch deviants in this input. The mean ISI was either 80, 160, 480, or 800 ms. Negative difference waves (Nds) were computed by subtracting ERPs to unattended standards from ERPs to the same stimuli when attended. The exogenous N1, as estimated from unattended standard ERPs, was larger contralaterally to the stimulus and inverted in polarity at mastoids. With decreasing ISIs, N1 diminished in amplitude much faster than did the Nd. In addition, N1 latency remained stable, whereas Nd peaked markedly earlier with shorter ISIs, almost perfectly coinciding with the exogenous N1. However, this temporal coincidence found in grand averages proved to be illusory in single subjects. The early Nd showed no contralateral asymmetry at its peak, but asymmetry was apparent during the ascending slope. These lateral asymmetries resembled those of the exogenous N1 but occurred later. The early Nd peak was, at least mainly, caused by an endogenous attention effect, the processing negativity (PN), even with very short ISIs, but an effect on the exogenous N1 could not be excluded.

Adult↗

PubMed1993

Intermodal selective attention: evidence for processing in tonotopic auditory fields.

Auditory event-related brain potentials (ERPs) were recorded for 250- and 4,000-Hz tone bursts in an intermodal selective attention task. Tonotopic changes were evident in the scalp distribution of the rising phase of the auditory N1 (mean peak latency 116 ms); the N1 was more frontally distributed following the 4,000-Hz than following the 250-Hz tone bursts, and it included a contralateral P90 component that was absent following 250-Hz tones. ERPs related to intermodal selective attention were isolated as negative and positive auditory difference waves (Ndas and Pdas). Neither the Nda nor the Pda showed changes in distribution with tone frequency, but both showed Ear x Frequency changes in distribution. ERPs for deviant tones included mismatch negativities (MMNs) and, in attend auditory conditions, N2b and P3 components. These components did not change in scalp distribution with tone frequency. One possible explanation is that tonotopic displacements of ERP distributions on the scalp surface depend on angular displacements in generator fields on gyral convexities. The results are consistent with the possibility that auditory processing radiates outward with increasing latency from tonotopic fields on Heschl's gyri to more gyrus-free regions of the planun temporale and anterior superior temporal plane.

Acoustic Stimulation↗

PubMed1992

Intermodal selective attention. I. Effects on event-related potentials to lateralized auditory and visual stimuli.

The effects of intermodal selective attention on event-related brain potentials (ERPs) were examined in 2 experiments. In experiment 1, auditory ERPs were compared (1) when subjects responded to easy and difficult-to-detect target tones in sequences of tone bursts; and (2) when they ignored the same auditory sequences and played a demanding video game. In experiment 2, auditory ERPs to tone bursts and visual ERPs to vertical line gratings were compared as subjects responded to difficult-to-detect targets in one modality or the other. Attention to auditory stimuli resulted in biphasic enhancements in auditory ERPs, the Nda (negative auditory difference wave, latency 120-160 msec) and the Pda (positive auditory difference wave, latency 200-240 msec) waves. These had longer latencies and somewhat different scalp distributions than N1 and P2 components evoked by non-attended tones. The Nda and Pda could be contrasted with the monophasic processing negativities typically found in dichotic selective attention tasks. Nda amplitudes were larger for difficult-to-detect targets (closely resembling standards) than for standards themselves, but no Ndas were recorded to highly deviant targets. Deviant auditory stimuli evoked mismatch negativities (MMNs) that persisted during visual attention. MMN amplitudes to difficult-to-detect deviants were enlarged with attention, but no change was found in MMN amplitudes to easy-to-detect deviants. In experiment 2 intermodal attention was associated with biphasic changes in visual ERPs over the posterior scalp: the occipital Pdv (100-130 msec), and contralateral-temporal Ndv (120-320 msec) deflections. Deviant visual stimuli also elicited mismatch negativity/N2b components, largest over the inferotemporal cortex contralateral to the stimulated visual field. Like the auditory MMN, the MMN increased in amplitude with attention, but it was also evident during attend auditory conditions. The results suggest that sustained, intermodal attention depends primarily in processing modulations in modality-specific cortex. We found no evidence of the participation of modality non-specific cortex. This excludes the possibility that intermodal attention depends on a single, supramodal attention system. The relatively long latency of intermodal effects suggests that they may depend on the reafferent (top down) modulation, and do not index "template matching" operations.

Acoustic Stimulation↗

PubMed1992

Intermodal selective attention. II. Effects of attentional load on processing of auditory and visual stimuli in central space.

The effect of processing load on event-related brain potentials (ERPs) was investigated in an intermodal selective attention task in which subjects attended selectively to auditory or visual stimuli. Processing load was manipulated by requiring subjects to detect either difficult-to-detect (deviant) or easy-to-detect (DEVIANT) targets in separate blocks of trials. Attention to auditory stimuli was associated with negative (Nda, 90-170 msec) and positive (Pda, 190-270 msec) enhancements in the ERPs to auditory stimuli. The Nda increased in amplitude with increasing processing load. Deviant auditory stimuli occurring among auditory standard stimuli elicited frontally distributed mismatch negativities (MMNs). The MMN persisted during visual attention and was unaffected by visual processing load. However, the MMN to deviants but not DEVIANTS was enhanced in amplitude with auditory attention. Attention to visual stimuli resulted in positive (Pdv, latency 70-130 msec) and negative (Ndv, 170-270 msec) modulations of visual ERPs, that increased with increasing processing load. Prominent visual deviance-related negativities were observed at occipital and infero-temporal scalp sites (latencies 90-290 msec), but only to DEVIANT visual stimuli. The early MMN-like portion of the visual deviance-related negativity was independent of attention, with equal amplitudes during different auditory and visual conditions.

Acoustic Stimulation↗

PubMed1992

Event-related potentials to repetition and change of auditory stimuli.

The major intent of this study was to compare the role of stimulus repetition and change in the elicitation of the MMN, an ERP component specific to stimulus change, and N2b, usually partially overlapping the MMN when stimuli are attended. Event-related potentials were recorded in one set of conditions where subjects ignored the stimuli and read a book, and in another set of conditions where subjects counted stimuli designated as targets. Stimuli were delivered in 4 ways, the common feature between all these conditions being the occurrence of infrequent events at a probability of 0.20: (1) an oddball paradigm with 1 deviant, (2) an oddball paradigm with 2 deviants, each with a probability of 0.10, (3) a regular alternation of tones of 2 pitches where either of the 2 tones infrequently repeated (P = 0.20), and (4) a random presentation of tones of 5 different pitches, where any of the 5 tones infrequently repeated (P = 0.20). In the count conditions, the infrequent events were designated as targets. It was found that the MMN was elicited by stimulus change and not stimulus repetition in the ignore and count conditions, whereas the N2b was elicited by both stimulus changes and repetitions in the count conditions. It was also possible, in the count conditions, to disentangle the part of the late positive complex which is related to stimulus deviation and the part which is related to stimulus significance (target).

Acoustic Stimulation↗

PubMed1992

Auditory attention and selective input modulation: a topographical ERP study.

Event-related potentials (ERPs) were recorded in subjects receiving tones (left ear 300 Hz, right ear 6000 Hz) at a rapid rate and trying to detect occasional higher-pitched stimuli in a designated ear. ERPs to attended stimuli showed enhanced negative amplitudes whose topographical distribution differed from that of the exogeneous N1 component. Moreover, the latter was considerably larger for low than high tones, whereas the attention effect had similar amplitudes for the two tones. Consequently, the attention effect, even when perfectly coinciding in time with N1, does not seem to be caused by modulation of the exogeneous N1 but rather by a separate process activated by attention. This suggests that attention does not modulate initial stimulus representations in audition.

Acoustic Stimulation↗