Perceptual anchoring to linguistic and non-linguistic stimuli in participants with acquired brain lesions

This project will examine the cognitive process of perceptual anchoring in response to auditorily presented non-linguistic (pitch discrimination) and linguistic stimuli (phoneme discrimination) in participants with unilateral circumscribed acquired brain-lesions and a neurotypical otherwise matched control group. We include equal numbers of participants with single either right or left hemisphere lesions. Using dichotic presentation (signal vs. noise) either to the left or right ear, lateralisation issues will be addressed. We formulate three research questions: (i) does anchoring for the non-linguistic and linguistic paradigm correlate or dissociate between left- and right ear presentation; (ii) do brain lesions modulate perceptual anchoring and does this depend on the lesion side (left versus right hemisphere); (iii) does the patholinguistic profile correlate with modulations of anchoring in people with an acquired brain lesion. The experiment will comprise two tasks: A pitch discrimination task and a selective adaptation task which modulates the perception of the phonetic boundary between /b/ and /d/. In the non-linguistic pitch discrimination task, participants hear 70 pairs of tones per block, and are asked to indicate which tone is higher. There will be an anchor condition, in which the first tone will always be 1000 Hz and the second tone will be chosen based on the adaptive procedure. This is contrasted to a no-anchor condition, in which both tones will vary in frequency, with the first tone being randomly chosen from a frequency range of 800-1250 Hz and the second tone being determined based on the adaptive procedure. A two-down, one-up adaptive staircase procedure will be used to target 79% correct on the psychometric function (Levitt, 1971), with step size decreasing every 4 reversals from 50% to 28.6% to 9.1 % to 4,8% of the previous ratio of frequency difference. Initial frequency difference between tones will be 50% of the frequency of the first tone (log-transformed). Tone duration will be 40 ms and interstimulus interval (ISI) will be 500 ms. Before the start of the main experiment, participants will perform a test run to ensure correct understanding of the task and sufficient hearing performance. On each ear, 10 anchor-trials will be presented with the starting frequency difference of the main experiment (i.e., 50% of the frequency of the first tone, log-transformed). In the selective adaptation task (= linguistic anchoring) a continuum of 20 syllables between the endpoints /ba/ and /da/ created by Stephens and Holt (2011) will be used. In an initial pre-test at the beginning of each selective adaptation task, participants will be presented with each step of the continuum twice (i.e., 40 syllables in total). They will perform a forced choice decision regarding their perception of /ba/ or /da/. This pre-test will be followed by four rounds of an adaptation-test procedure: Participants will hear 70 instances of the prototypical endpoints of the continuum (either /ba/ or /da/). This has been shown to shift the phonetic boundary closer to the prototypical exemplar. In the ensuing identification phase the 18 intermediate steps of the continuum will be presented and participants will again perform the forced choice decision. Both adaptors (/ba/ and /da/) will be presented alternatingly (two times each per block), with the first two selective adaptation blocks starting with one adaptor and the last two selective adaptation blocks starting with the second adaptor. Stimuli will be presented dichotically (signal/white noise). The structure of the experiment will be the following: • 4 blocks of the selective adaptation block (ear 1) • Frequency discrimination block (ear 1, anchor or no-anchor) • 4 blocks of the selective adaptation block (ear 2) • Frequency discrimination block (ear 2, anchor or no-anchor) - clinical testing - • 4 blocks of the selective adaptation block (ear 1) • Frequency discrimination block (ear 1, anchor or no-anchor) • 4 blocks of the selective adaptation block (ear 2) • Frequency discrimination block (ear 2, anchor or no-anchor) Order of ears, first adaptor in the selective adaptation task and of anchor and no-anchor blocks will be counterbalanced across participants for each participant group (participants with right-hemisphere lesions, participants with left-hemisphere lesions, control participants).