E. BUCKOLZ ET AL.
exhibited significant inhibitory after-effects. On this basis, and
having explained dissenting results, they concluded that IOR
and SNP had comparable causes. It is possible, however, that
the target-target slowing observed by Christie and Klein oc-
curred because they used an SNP-peripheral procedure. This
possibility is supported by the fact that a target-repeat facilita-
tion rather than an interference effect is observed with central
locations in SNP tasks. This held both when target plus dis-
tractor (e.g., Buckolz et al., 2008; Guy et al., 2006; Fitzgeorge,
2009; Fitzgeorge & Buckolz, 2008) or target-only probe trials
had been employed (e.g., Guy et al., 1994). So, target-repeat
data do not implicate the involvement of orientation or location
inhibition for centrally delivered events. Hence, they do not
argue for IOR and SNP-central having a common production
Also bear in mind that the Christie and Klein (2001) data did
not rule out the involvement of response inhibition (Fitzgeorge
et al., 2011) as a contributor to their SNP effects. So, on this
account as well, their data do not unequivocally point to a
common mechanism for IOR and SNP-central.
The Vector (Center of Gravity) Model of IOR
Klein et al. (2005) were the first to report that when simulta-
neously presented distractor events were symmetrically posi-
tioned on either side of midline in the visual periphery, IOR
effects failed to materialize at these stimulated locations
(Fitzgeorge & Buckolz, 2009). This failure is further evidence
that distractor-occupied locations are not themselves inhibited,
albeit peripheral locations in this case. Additionally, these
failed IOR effects have altered the typical IOR explanation, a
possibility that needs to be acknowledged in the IOR vs
SNP-central distinct phenomena debate. The account is ap-
proximately as follows.
In the usual IOR procedure, a single exogenous stimulus
generates a vector (i.e., an orienting response) which is the
source of inhibition and which points to the stimulated position.
Alternately, paired stimulations produce a net vector that is
positioned midway between the two actual stimulation posi-
tions, which then serve as the center of inhibition, and from
which radiates a decreasing gradient of inhibition magnitude.
This manoeuver separates the true source of inhibition (i.e., the
net vector) from the cued locations. In so doing, it reveals that
distractor-occupied (cued) locations do not yield inhibitory
after-effects, which means that they are not themselves inhib-
ited (i.e., the IOR effect; RT[cued] = RT[uncued]). Hence, nei-
ther IOR nor SNP-central effects are caused by location inhibi-
tion. However, their causes would differ in that a cued periph-
eral location would generate an inhibitory vector, while it is
unlikely central stimulations do the same. This is supported by
the RT(DRR) > RT(IR) finding noted earlier (Edgar, 2011; Guy
et al., 2006), showing the prime distractor location unrelated to
Finally, if IOR and SNP-central effects have a common
cause, the net vector influence should cause SNP-central values
to be smaller with target plus distractor than with distractor-
only prime trials. That is, different-side prime-trial presenta-
tions should have produced little or no measured inhibitory
after-effects1, reducing the SNP calculated value. This influ-
ence would be absent in with distractor-only primes. This did
not occur (Buckolz et al., 2008).
In sum, the existing literature indicates that the IOR and
SNP-central effects have distinct causes; the former generating
inhibition from peripheral location processing (inhibitory vector
“our term”/orientation inhibition) while, with the SNP-central
task, distractor-response inhibition solely produces the inhibi-
tory after-effects observed.
The Current Experiment
Four tasks were used, differing with regard to event location
(central [C], peripheral [P]) and to manual response number
(1-response [1-R] vs 4-response [4-R]). Prime trials contained
target or distractor event, the probe trial only the former (Buck-
olz et al., 2008). Theoretically, these Tasks represented a con-
tinuum of inhibition producing mechanisms: Task 1(C[1-R]) =
none (past research) or potential orientation inhibition/ vector
inhibition (to-be-tested), Task 2 (P[1-R]) = orientation inhibit-
tion, vector inhibition (IOR), Task 3 (C[4-R]) = response inhi-
bition (SNP-central), and Task 4 (P[4-R]) = orientation inhibit-
tion/vector inhibition + response inhibition.
Importantly, Task 1 can provide direct evidence that cen-
trally stimulated locations do not generate inhibitory after-ef-
fects (since this Task lacks the response processing held to
produce the SNP effect). This evidence would take the form of
the absence of an inhibitory after-effect altogether with this
Task, or if an observed inhibitory after-effect is accompanied
by equivalent RTs for target-repeat (TR) and control (CO) trials.
The latter follows from Coward, Poliakoff, O’Boyle, & Lowe
(2004). They proposed that the distractor-occupied location
forms a stronger bond with the location’s activated and subse-
quently inhibited response so that this response inhibition ex-
erts a stronger influence when the probe target occupies the
prime distractor location. This contributes to the production of
the IOR effect on distractor-target trials, a contribution that is
removed on target-repeat trials leaving only orientation inhibit-
tion to operate. Hence, an RT(target-repeat) = RT(control)
finding points to the absence of the latter inhibition type. The
uncertainty at this point is whether the proposal of Coward et al.
operates with central event presentations.
Furthermore, Task 4 (P[4-R]) will allow us to study the in-
teractive effects of orientation and response inhibition subse-
quent to distractor primes, comparing its after-effects with
those of Tasks 2 (P[1-R]) and 3 (C[4-R]). Also, following tar-
get primes in Task 4, we can examine opposing positive (repe-
tition) and negative (orientation inhibition) forces on latency
production. The RT(target-repeat[TR]) vs RT(Control[CO])
relationship will indicate which force, if any, prevails. Impor-
tantly, should RT(TR) < RT(CO) occur (Chao, 2009), the prac-
tice of using this inequality to signal the absence of orientation
inhibition would have to be discontinued.
1We were able to examine different-side target-plus-distractor prime trial
presentations with some SNP-central pilot data that used a 2.0 inter-trial
delay (n = 22) and target-plus-distractor probes. A significant SNP-central
effect (21 ms) was obtained, (t = 2.64, p < 0.02, SD = 37.62), indicating
that central distractor locations do not generate inhibitory net vectors; oth-
erwise the SNP effect should have been absent. Hence, IOR and SNP-central
have different causes on this account.
Forty university undergraduate students, ranging in age from
20 - 30 years and with normal or corrected-to-normal vision,
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