The effect of meaning entropy during auditory homophone recognition: MEG evidence for amodal lexical entries
Gwyneth Lewis1 & Alec Marantz1,2
New York University: Departments of Psychology1 & Linguistics2
Introduction
Methods (continued)
MEG studies report correlations between superior temporal (ST) activation at ~350ms poststimulus onset (the MEG M350) and the lexical properties of visually presented words
M50
Subjects

(Solomyak & Marantz, 2009, 2010; Lewis et al., in press)

five males, five females
right-handed native English speakers with structural MRIs
In recent work the meaning entropy (or degree of ambiguity) of visually presented
homophones — orthographically and phonetically identical words with distinct meanings —
modulated the M350 response but not earlier responses (which were modulated by form
properties of the stimuli) (Simon et al., in press).
Experiment
The entropy effect — more activation for higher entropy — is understood on the
assumptions that 1) homophones have distinct entries, and 2) entries compete for
recognition.
Homophones with greater frequency balance between meanings have higher entropies and
thus yield more competition.
 balanced: “pawn”
 unbalanced: “down”
Higher entropy = more competition = stronger activation
Brain Data
If the M350 indexes activation of amodal representations, then the homophone entropy effect
for auditory homophones should occur in the same location and show similar temporal
properties to that produced by visually presented homophones.
Independent Variables
auditory lexical recognition task
stimuli presented across five randomized blocks
 MEG data acquired continuously throughout the experiment.


M100
structural MRIs reconstructed in FreeSurfer
MEG data averaged over all trials for each subject
 activation within FreeSurfer transverse temporal label
 minimum norm solutions computed for each label, for each
subject, for each trial in the raw data.


Average activation in LH transverse temporal ROI
M200
Dependent Variables
response time
 trial-by-trial ms-by-ms ROI activation.

Time from stimulus onset (ms)
residual entropy (from a linear regression that removed the effect
of surface frequency)
 surface frequency

Methods
Time from stimulus onset (ms)
Results
Stimuli
Effects of residual entropy
Mac OS X text-to-speech generated the speech stimuli from the 500 Simon et al. (in press)
study, each associated with multiple meanings and senses.
As in previous studies on homophones, the number of senses for each meaning was used to
estimate the frequency of that meaning.
Variables
number of meanings (M) as count of dictionary headword entries
 number of senses within each meaning (Sm)
 length in characters
 summed bigram frequency
 log frequency in HAL corpus
 entropy (u) based on relative frequencies of meaning for each word (Mn) and number of
senses for each meaning (Sn,m).

Visual
Example: “quack” 27
Meaning 1: senses
1. noun: The sound made by a
duck.
2. intrans. verb: To utter a quack.
Meaning 2: senses
1. noun: Someone who practices
medicine incompetently
2. adj: Of, concerning, being, or
characteristic of a quack.
3. intrans. verb): To act as a quack.
Mean
SD
Length
Variable
4.22
0.88
Bigram freq.
2141
1101
Surface freq.
8.66
1.478
Meanings
2.23
0.54
Senses
8.22
4.69
Entropy
0.93
0.31
Correlations with RT
Variable
1. RT
word recognition (Simon et al., in press)
Recall that in visual word
recognition, higher
entropy is associated with
stronger ST activation at
~300ms post-stimulus
onset (Simon et. al., in
press).
2. Entropy
~300ms
3. Surface freq
*p < .05
4. Bigram freq
5. Meanings
1
---
2
-0.01
3
-0.05
--- **-0.17
---
4
-0.01
5
-0.03
6
-0.02
-0.01 **0.81 *-0.03
0.18
---
-0.02 **0.47
-0.02
7
8
-0.02 **-0.18
0.01
-0.01
-0.01
**0.13
**0.06
0.03
0
--- **0.23
0
0.01
---
-0.01
**0.07
---
-0.03
6. Senses
7. Duration
8. Uncertainty
---
* p < .05, **p < .01
Conclusion
Average activation in functionally defined LH
superior temporal label from Simon et al. (in press).
Auditory
Entropy formula
Grand average of raw MEG data
word recognition (present study)
~300ms
We found an entropy effect
similar to the latency and
location of Simon’s effect
— higher entropy yielding
stronger activation at
~300ms post-stimulus
onset.
Average activation in anatomically defined LH
transverse temporal label from the present study.
*p < .05
*p = .05 significance level following correction for multiple comparisons. The
bold line identifies temporal clusters subject to the Monte-Carlo correction
procedure over the 250-350ms time window. An effect of entropy was found
between 252-297ms (Σr = 1.5794 for 46 time points, p =0.012 following
correction for multiple comparisons),
Transverse temporal ROIs yielded negative ST activation at
~300ms (as in Simon et al.). Activation from these temporal
ROIs correlated significantly with the entropy variable at ~300
ms, similar to the latency and direction of Simon’s effect, and in
the same ROI — greater entropy yielded greater activation,
consistent with the lexical competition model. The M350 reflects
similar resolution stages in auditory and visual word
recognition whereby amodal lexical entries compete for access,
comparable to the parallel auditory and visual N400m ST
responses seen in MEG (Helenius et al., 2002).
References
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