Anthony Herdman

Associate Professor

Research Classification

Auditory System
Visual System
Audiovisual, Visual, Audio and Written Communications
Language and Cognitive Processes

Research Interests

Neuroimaging Methods (EEG/MEG)
Central auditory processing
Auditory and visual perecptions related to reading acquisition (1st and 2nd languages)
Brain computer interface

Relevant Degree Programs

Affiliations to Research Centres, Institutes & Clusters


Research Methodology

Electroencephalography (EEG)
Magnetoencephalography (MEG)


Master's students
Doctoral students
Postdoctoral Fellows
I support public scholarship, e.g. through the Public Scholars Initiative, and am available to supervise students and Postdocs interested in collaborating with external partners as part of their research.
I support experiential learning experiences, such as internships and work placements, for my graduate students and Postdocs.
I am open to hosting Visiting International Research Students (non-degree, up to 12 months).
I am interested in hiring Co-op students for research placements.

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2021)
Neural underpinnings of orthography in first and second language readers (2019)

Reading success in a second language (L2) is crucial for billions of people worldwide, but it is well known that it remains difficult for L2 learners to automatize the L2 processing in general. I investigated how long-term reading experience in a first language (L1) might change the neural L1 orthographic processing, and if these results can be generalized to L2 orthographic processing. I recruited adult monolingual English readers and adult Mandarin readers with late-language learning of English. By contrasting single-letters with pseudoletter visual stimuli (a pseudoletter effect) in L1, the reaction time data showed that reading experience makes letter processing faster than in pseudoletter processing. The electroencephalogram data showed that the L1 pseudoletter effect was manifested in a left-dominant oscillatory activity and network dynamics. The electroencephalogram data also showed that the L1 pseudoletter effect was robust, regardless of the level of attention paid; letters elicited more of left-lateralized neural connectivity desynchronization than did pseudoletters. Additionally, the data from L2 showed that the magnitude of the L2 pseudoletter effect in the N170 in the left hemisphere was correlated with L2 proficiency. Taken together, I concluded that with reading experience, the brain has automatized orthographic processing, which is evidenced by being (1) more specific by shifting the processing demands to different neural regions within visual processing networks—left dominant for well experienced orthographies, (2) being more obligatory, at least, at the single-letter level, and (3) faster by completing the process of differentiating letters from pseudoletters at fairly early stages of visual processing. Further, I concluded that some of the above changes appear to be ready throughout adulthood; orthographic development appears to be free from the fossilization or critical period hypotheses.

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Master's Student Supervision (2010 - 2020)
The effect of noise floor on cortical auditory evoked potential response detection at threshold intensities (2019)

Cortical auditory evoked potentials (CAEPs) are neural responses that occur in response to changes in sound, which can be recorded from electrodes placed on the scalp. The N1-P2 response can be reliably used to determine a person’s frequency-specific hearing thresholds. This objective method for hearing assessment is used clinically in situations where the patient is unable or unwilling to provide reliable behavioural responses. Currently, the gold standard method of interpreting CAEP results is dependent upon the visual judgment of the clinician. A high level of noise in the recordings may obscure a small N1-P2 response. Established criteria for an acceptable residual noise (RN) level does not currently exist. Such criteria could be used as a tool to assist in the interpretation of CAEPs, making judgments more reliable among clinicians. The goal of the present study was to estimate a noise criterion based on recorded and simulated CAEP averages with various different levels of RN.CAEP results at threshold and sub-threshold intensities were recorded for 12 normal hearing adults at 500 Hz and 2000 Hz. Each waveform was presented a total of five times, with each average including a different number of sweeps. Simulated CAEPs were generated (N=37), with averages including a variety of different RN levels. The waveforms were presented to three expert raters, who judged each waveform independently as having an N1-P2 response present or absent. RN criterion, maximal noise floor allowable for judging a response as truly absent, was determined when raters performed at 95% sensitivity of detecting a true response. The recommended criteria for the maximum noise floor level were 0.111 μV (relative to the pre-stimulus interval) or 0.145 μV (relative to the post-stimulus interval). Simulated data exhibited a better face validity than the recorded CAEPs. Further studies may lead to the implementation of new guidelines surrounding RN criteria in CAEP recording procedures. Such guidelines may provide more validity and reliability in clinical practice.

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Cochlear microphonics from auditory brainstem responses of infants with auditory neuropathy spectrum disorder (2018)

Objectives: The cochlear microphonic (CM) is a bioelectric potential detectable through early latency auditory brainstem responses (ABR). It has been described as large in amplitude and long in duration in auditory neuropathy spectrum disorder (ANSD), a complex form of hearing loss. ANSD is identified through a combination of diagnostics, including but not limited to present otoacoustic emissions (OAEs) and/or a robust CM, and absent/abnormal ABR. Based on previous literature, this study hypothesized that CMs would differ significantly between infants with ANSD and normal-hearing infant data from well-baby, and neonatal intensive care unit (NICU) populations; it further hypothesized that CM/ABR wave V (CM/V) amplitude ratios would not differ significantly across ANSD groups presenting with and without OAEs. Design: Retrospective analysis was performed on click-ABR recordings from 16 infants with ANSD (24 ears; mean 3.5 months, corrected) for comparison to published normative data. ANSD was identified by the presence of OAEs (OAE+) and absent/abnormal ABR, and by CM presence with abnormal/absent ABR and support from later behavioral results for infants with absent OAEs (OAE-). Waves were analyzed by comparing condensation and rarefaction polarities to highlight the CM. Proposed CM/V ratio values for ANSD identification were also explored. Results: Mean CM durations were significantly longer in combined ANSD ears (4.197 ± 1.154ms) than normally-hearing well-baby (0.73 ± 0.3ms) and NICU (0.82 ± 0.51ms) infants. CM amplitudes were significantly larger in ANSD (0.322 ± 0.173µV) than well-baby (0.24 ± 0.09µV), but not NICU (0.26 ± 0.13µV) infants. OAE+ and OAE- ANSD ears did not differ significantly in CM duration or amplitude but did differ significantly in mean CM/V ratios (6.602 ± 2.987, 2.040 ± 1.112, respectively). Ratios correctly identified ANSD in 16 of 19 ANSD ears with a wave V. Conclusions: Significant group differences in CM duration suggest that this measure could be useful for identification of ANSD in infants. CM amplitude was less definitive, with confounds between datasets. The CM/V ratios failed to correctly categorize all OAE- infants for which the measure would be most applicable. Results should be viewed with caution given the retrospective nature and limited sample size of the analysis.

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Comparative analysis of auditory brainstem response and hearing aid fitting thresholds in auditory neuropathy spectrum disorder (2018)

Objectives: In cases of auditory neuropathy spectrum disorder (ANSD), clinicians may defer amplification until reliable behavioural thresholds can be obtained, as auditory brainstem response (ABR) assessment often overestimates thresholds. Limited research has investigated the correlation between ABR-estimated and behavioural thresholds. This study aims to retrospectively quantify the management practices found in ANSD patients identified through the British Columbia Early Hearing Program (BCEHP). The relationship between ABR and hearing aid (HA) fitting thresholds will be investigated for evidence to support using ABR thresholds to inform early intervention. Methods: BCEHP patient data was obtained for the 28 children diagnosed with ANSD between 2008 and 2015. Information such as clinical demographics and the use of hearing technology was extracted. The age of diagnosis and intervention was reviewed for 1-3-6 month benchmark compliance. ABR and HA fitting threshold data was collected, and comparative analysis was performed. Results: Analysis revealed that the average age of ANSD diagnosis was 7.8 months corrected (range: 1.5 – 69 months), and the average time between diagnosis and amplification was 10 months (SD: 15 months). Comparative analysis revealed that while delaying amplification is appropriate for the majority of ANSD patients, fitting to ABR-estimated thresholds may be suitable in a small subset. Conclusions: HA fitting for ANSD infants is often delayed beyond 6 months of age. Study results suggest that in some cases, ABR thresholds may be sufficient to generate prescriptive targets for amplification. Further research is needed to corroborate these findings and provide support for early intervention based on ABR-thresholds.

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Effects of attention on cortical auditory evoked potentials during a gap detection task (2018)

Gap-detection testing is a measure of temporal resolution, a component of auditory processing that is sometimes affected in Central Auditory Processing Disorder (CAPD). Using behavioural gap-detection tasks as part of a CAPD battery can be confounded by attention, in that it is difficult to distinguish whether poor performance is a result of attention deficits or central auditory processing deficits. Researchers have assessed the utility of cortical auditory evoked potentials (CAEPs) as an objective measure of gap-detection thresholds. The present study aimed to compare CAEPs during a passive and active gap-detection task to assess the effects of attention on electrophysiological gap-detection testing. The results showed no significant differences in gap-detection thresholds obtained behaviourally, obtained electrophysiologically during the passive task, and obtained behaviourally or electrophysiologically during the active task. N1-P2 amplitudes were found to be significantly larger in the active gap-detection task compared to the passive task. In conditions where a 0 ms duration or subthreshold duration gap was presented, an N2b waveform was evoked. The N2b is an endogenous event-related potential (ERP) that is usually evoked when a prepotent response must be withheld, such as during the no-go trials of a go/no-go task. Because the gap-detection task could be considered as a go/no-go paradigm, the N2b was likely evoked due to the unfulfilled expectation of a gap occurring with predictable timing and high probability. The goal of experiment 2 was to investigate the occurrence of this endogenously-evoked no-go N2b as previous literature has focused primarily on the N2b evoked by exogenous stimuli. The results found that all participants had an N2b wave to no-gap and subthreshold gap conditions. There were no significant correlations between N2b-P3a amplitudes and behavioural thresholds or between N2b-P3a amplitudes or N2b latencies and reaction time or reaction time standard deviation. There was a significant correlation between P3a amplitudes and ex-Gaussian reaction time standard deviation. No significant differences were found between N2b amplitudes for correct rejections or misses. Overall, this study demonstrated that the no-go N2b can be evoked by an endogenous signal in the form of the omission of an expected gap in noise.

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Effects of hearing aid processing on cortical auditory evoked potentials in normal hearing individuals (2018)

Cortical auditory evoked potentials (CAEPs) are currently being investigated as a tool for validation in hearing aid fittings. There is some conflicting evidence regarding the usefulness of CAEPs in this capacity. CAEPs are influenced by stimulus parameters and hearing aids can change these parameters in an unpredictable manner. The purpose of this study was to investigate the effect of rise time after hearing aid processing on the CAEP of 23 normal hearing participants. Two different duration stimuli (60 ms and 120 ms) were processed by three different hearing aids and the output of each hearing aid was recorded. The stimulus parameters were measured for each condition and the stimuli were presented to each participant through an insert earphone. Two blocks of stimuli were used (1) Raw (varied SNR and intensity) and (2) Equalized/Filtered (equalized SNR and intensity). The electroencephalography (EEG) was recorded and the P1-N1-P2 amplitudes and latencies were measured for each condition. A three-factor ANOVA was conducted to observe the effects of (1) rise time, (2) duration, and (3) SNR. A main effect of rise time was observed on the N1-P2 amplitude. This result indicated that hearing aid processing can increase the rise time enough to elicit a decrease in the N1-P2 amplitude. No effects were observed on amplitudes or latencies of the N1-P2 with the alternative stimulus parameters (SNR and duration). Prior to using CAEPs clinically for validation of hearing aid fittings, normative standards should be established. This ensures that differences in the N1-P2 amplitudes are due specifically to audibility and not to the altered stimulus parameter (i.e., after hearing aid processing). Further research should also be conducted on individuals with hearing loss to see if the effects observed in this study would be present with this population. In addition, comparisons of behavioural and CAEP methods of validation would be helpful in determining the validity and reliability of using these methods clinically.

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Cortical auditory evoked potentials to estimate gap-detection thresholds in adults (2016)

Temporal resolution is the ability of the auditory system to detect small changes over time and is an important component for the detection and decoding of speech by the central auditory nervous system. Temporal resolution is most often measured by asking a person to detect small gaps between sounds, known as behavioural gap-detection tests. However, certain populations may be unable or unwilling to respond reliably due to perceptual or cognitive deficits, or in medico-legal compensation cases. There are limitations to behavioural gap-detection measures because they cannot separate cognitive from perceptual deficits. The present study utilized electrophysiological gap-detection measures as a means of objectively estimating behavioural gap-detection thresholds. Cortical auditory evoked potentials (CAEPs) are neural responses to changes in sound, duration, and frequency that can be measured from the scalp. The specific aim of this study was to collect adult normative data for CAEP gap-detection thresholds to examine if CAEPs could accurately estimate behavioural gap-detection thresholds. Gap-evoked CAEPs could be recorded in participants who were awake and passively listening and could estimate temporal resolution without the need for a participant’s cooperation or attention. The results showed that there was a significant N1-P2 response to gaps at ≥ 4 ms, with 85% of participants having a response to a 10 ms gap. Additionally, the electrophysiological mean gap-detection threshold was within 1 ms of behavioral mean gap-detection threshold. This demonstrated that gap-evoked CAEPs can accurately estimate behavioural gap-detection thresholds in a normal hearing adult population.

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Behavioural Measures of Interference and Facilitation in an Audiovisual Colour-Word Stroop Matching Task (2015)

Previous audiovisual Stroop studies used spoken colour-words mainly as ignored distractors when performing the visual Stroop task. Previous matching Stroop studies in the visual domain provided opposing views regarding whether interference effects resulted from conflicting semantic representations or conflicting responses. This study’s main objective was to explore how a written word distractor affects audiovisual matching of a spoken colour-word and font colour. I presented colour-words written in congruent or incongruent font colours simultaneously with spoken colour-words. Participants pressed buttons to indicate whether the spoken word and font colour were “same” or “different”, while ignoring written word meaning. I recorded response times and accuracy to measure interference and facilitation effects between experimental and control conditions. I hypothesised that incongruent written words (e.g., red) would interfere with “same” responses (e.g., font colour = blue, spoken word = /blue/) but facilitate “different” responses (e.g., font colour = green, spoken word = /blue/); and that congruent written words (e.g., red) would facilitate “same” responses (e.g., font colour = red, spoken word = /red/) but interfere with “different” responses (e.g., font colour = red, spoken word = /blue/, or font colour = blue, spoken word = /red/). The results showed large interference effects but no facilitation effects on audiovisual judgements. While incongruent written words interfered with “same” responses, congruent written words interfered with “different” responses. The largest interference effect occurred when the written word was incongruent with both task-relevant dimensions, while smaller effects occurred when the written word was congruent with either task-relevant dimension. Consistent with previous Stroop findings, my audiovisual matching task showed that in the case of cross-modal colour judgements, written word meaning predominantly interferes with but does not facilitate performance. The pattern of results showed that a conflict between the outcome of the relevant matching task and the outcomes of two mistakenly performed matching tasks involving the written word produced interference effects, rather than a conflict among the semantic representations activated by the three stimulus dimensions.

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An electrophysiological investigation of auditory temporal processing (2013)

Infant hearing-health programs have a goal of identifying infants with a permanent hearing loss by the age of three months and treating these infants by the age of six months. However, deficits in hearing thresholds are not the only deficits that exist in the auditory system. The ability of an infant's auditory system to resolve rapid changes in acoustic signals (i.e., temporal resolution) and integrate acoustic information over time (i.e., temporal integration) is important for typical language development. Because behavioural responses are unreliable for diagnostic purposes before the age of six months, electrophysiological measures of temporal resolution and integration could be beneficial. The main objective of my thesis was to validate in adults if 80-Hz auditory steady-state responses (ASSRs, an objective electrophysiological measure) can be used to assess temporal resolution and integration. Physiological temporal resolutions of adults were estimated from cortical auditory evoked potentials (CAEPs) and ASSR resets evoked by gaps (1.5625, 3.125, 6.25, 12.5, and 25 ms) within 40-Hz and 80-Hz amplitude-modulated white-noise bursts. Physiological gap-detection thresholds for CAEPs (8 ± 6 ms, averaged across conditions), 40-Hz ASSR resets (6 ± 5 ms), and 80-Hz ASSR resets (5 ± 4 ms) were comparable to behavioural gap-detection thresholds (5 ± 2 ms). However, 40- and 80-Hz ASSRs maximally reset to half-cycle gap durations (i.e. 12.5 and 6.25 ms respectively), thus ASSR resets might not be truly measuring gap-detection thresholds. Conflicting results from CAEPs and ASSR resets to gaps provides evidence that CAEPs respond preferentially to all gaps down to threshold; whereas, ASSRs preferentially reset to gaps that violate their modulation periodicity. Physiological integration times (117 ± 48 ms, averaged across conditions), as measured from the rise time of the ASSR resets, were comparable to behavioural measurements of temporal integration (132 ± 83 ms). However, more research is required to determine if physiological and behavioural integration times are correlated or are coincidentally similar. These results indicate that CAEPs are accurate measures of temporal resolution. However, further research is required to determine the utility of ASSR resets in assessing temporal resolution and integration.

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