Upload
truongkhanh
View
238
Download
0
Embed Size (px)
Citation preview
Visual short-term memory load disrupts subitizing limit
Simone Cutini, Francesco Sella & Marco Zorzi
Department of General Psychology, University of Padova, Italy
http://www.mendeley.com/profiles/simone-cutini/
ABSTRACT
Early visual processing is characterized by the parallel elaboration of a massive amount of information, nonetheless the number of objects that can be simultaneously
tracked and memorized is surprisingly limited, as observed both in visual short-term memory (VSTM) and non-symbolic visual enumeration tasks. Here we devised a
dual-task paradigm approach to address the nature and the selectivity of the link between VSTM and subitizing. Verbal memory load was used as control condition and
a stringent method to evaluate the individual subitizing range was employed. Our results demonstrate that VSTM load, but not verbal load, modulated performance
within the subitizing range without affecting the estimation of larger numerosities. This finding provides converging evidence regarding the presence of two distinct
mechanisms specifically associated to subitizing and estimation. Importantly, we found a striking correspondence between the number of elements retained in VSTM
and the decrement in the number of elements that can be subitized. In particular, the trade-off between VSTM load and enumeration accuracy at the subitizing limit
strongly suggests that VSTM and subitizing share the same cognitive resources.
METHOD
Participants: Twelve participants (5 males; Mean age = 23.6, SD = 3)
Verbal Memory Visual short-term memory
CONCLUSIONS
The present investigation aimed at unveiling the genuine nature of the relation between VSTM and subitizing. Our results demonstrate that the amount of VSTM load
modulates enumeration performance in the subitizing range without affecting the ability to estimate numerosity beyond the subitizing limit. Indeed, accuracy of enumeration
was affected by VSTM load only for numerosities around the subitizing limit (i.e., 3, 4 and 5 dots). This dissociation provides further evidence for the claim that subitizing
and estimation are related to two different mechanisms (e.g., Hyde, 2011; Piazza et al., 2011; Revkin et al., 2008) rather than being two extremes belonging to the same
continuum. Behavioral dissociation between subitizing and estimation was also highlighted in the study of Burr, Turi and Anobile (2010) as a function of attentional load
induced by dual task conditions (but see Vetter, Butterworth & Bahrami, 2008, for contrasting results). Moreover, different neural signatures for subitizing and estimation
have been highlighted in recent neuroimaging studies (Cutini et al., in press; Ester, Drew, Klee, Vogel, & Awh, 2012). More importantly, our results provide compelling
evidence with regard to the existence of a specific and selective link between VSTM and subitizing. We calculated the individual subitizing limit with a stringent procedure
and we found a remarkable correspondence between the number of elements retained in VSTM (as measured by Cowan’s K) and the decrement in the number of
elements that could be subitized. In particular, the trade-off between VSTM load and enumeration accuracy at the subitizing limit provides compelling evidence that VSTM
and subitizing share the same cognitive resources.
References: Burr et al. (2010). Journal of vision, 10, 1–10. Cutini et al. (in press). Neuroimage. Ester et al. (2012). The Journal of neuroscience, 32, 7169–77. Hyde, D. C. (2011). Frontiers in human neuroscience, 5, 150. Revkin
et al. (2008). Psychological science, 19, 607–14. Piazza et al. (2011). Cognition, 121, 147–53. Vetter et al. (2008). PloS one, 3, e3269.
RESULTS
http://ccnl.psy.unipd.it
We then retrieved for each participant the accuracy of enumeration at the individual
subitizing limit under the three different load conditions (VM, low-load VSTM, high-load
VSTM.
Enumeration accuracy at the individual subitizing limit, Repeated measure ANOVA with:
Load Type (Verbal, VSTM-Low, VSTM-High): F(2, 11) = 8.16, p = .002
Enumeration Accuracy (black line):
VM>VSTM-Low load (p = .035); VSTM-Low load>VSTM-High load (p = .022).
Cowan’s K (i.e., the number of items memorized in the VSTM task, gray line) was
significantly lower in the low-load VSTM (M = 0.82, SD = 0.11) as compared to the high-
load VSTM condition (M = 1.65, SD = .7), t(11) = 4.26, p = .01
Participants accomplished two different memory tasks
in a dual-task condition.
Verbal Memory (VM) task: four disyllabic pseudo-words
were presented through earphones for four seconds (1
second each).
low-load condition: one word repeated four times;
high-load condition: four different words.
VSTM task: a 2 x 2 memory array with four figures was
presented for 500 ms.
low-load condition: the 4 figures were identical;
high-load condition: the 4 figures were all different.
After the memory stimuli, a cloud of dots (numerosity
from 1 to 9) was shown for 200 ms and then replaced by
a mask that lasted for 100 ms.
Task 1: report the number of dots (keyboard).
Task 2:judge whether the test figure/word was present in
the memory array (Y/N).
Dots enumeration
VSTM load–subitizing trade-off
Omnibus ANOVA on dots enumeration accuracy: All main effects
and two-way interactions were significant; Memory Task x Load x
Numerosity, F(8,11) = 2.10, p = .044, indicating that dots numerosity and
load amount had a different impact depending on load type.
Accuracy in the low-load vs. high-load conditions for each numerosity
enumeration performance was worse in the high-load condition only for
numerosities around the subitizing limit (3, 4 and 5 dots, p<.05) .
Results suggest an Interplay between VSTM and subitizing.