Experiment 1: Fast mapping, retention, and generalisation of novel words in children with ASD and typical development
Method
Materials
Stimuli included four novel words (dax, wug, yok, lep) selected from the NOUN database (Horst & Hout, 2016), 11 familiar objects, and 12 unfamiliar objects (see Figure 1). Familiar objects were selected on the basis that most children understand their linguistic labels by 16-months (Fensen et al., 1994). Unfamiliar objects were selected on the basis that children would not know their linguistic labels. Three familiar objects (models of a dog, a flower, and a car) were employed in the warm up trials. The unfamiliar objects were divided into four sets. Each set included a ‘named object’ (which was paired with a novel word during referent selection trials) and a ‘shape match’ (a differently-coloured variant of the named object). Each shape match was the same colour as a named object from a different set, and therefore served as a ‘colour match’. The remaining eight familiar objects (miniature shoe, spoon, book, plastic tree, toy cow, plastic banana, children’s cup, toy horse) were divided into pairs and presented alongside unfamiliar named objects in the referent selection trials.   
Objects were presented at each stage of the word learning task via a specially-designed tray with three sections positioned to the left, centre, and right. The three sections were separated by 17cm gaps.

Named Object

Shape Match

Colour Match

Set 1

Set 2

Set 3

Set 4

Fig. 1 Sets of unfamiliar objects used in Experiments 1 and 2

Procedure
Participants were tested individually in their own schools and were accompanied by a familiar adult when required. Children were verbally praised for attention and good behaviour. Corrective reinforcement was only provided in the warm up trials. Children were administered the BPVS and Leiter-3 and then completed the word learning task 1-week later. The word learning task consisted of the following stages delivered in a fixed order: 1. warm-up trials, 2. referent selection trials, 3. test object familiarisation, 4. delay, 5. retention and generalisation test trials.
Warm-up trials. The task began with 3 warm-up trials. On each trial, children were presented with three familiar objects on different sections of the tray. The experimenter waited silently for approximately 3 seconds before asking the child to identify one of the objects (e.g. “Which is the car? Can you see the car? Show me the car.”) and sliding the tray forward. If the child was reluctant to respond, the experimenter provided up to 4 additional prompts per trial. Following a correct response, the experimenter issued praise and reinforced the identity of the object (e.g. “Great job, that is the car!”). Following an incorrect response, the experimenter provided corrective feedback (e.g. “Actually, this is the car. Can you touch the car? Well done, you touched the car!”). The experimenter then retrieved the tray and re-ordered the objects for the next trial. Children were asked to identify a different object (dog, flower, car) in a different location on the tray (left, centre, middle) in each of the 3 trials. 
Referent selection trials. Eight referent selection trials immediately followed the warm-up trials and had an identical format, except children did not receive feedback related to their responses (the experimenter just said “thank you”). Children were presented with 4 sets of objects, each consisting of one unfamiliar named object, and two familiar objects. Each set was presented twice; once the experimenter requested the familiar object (‘familiar name trial’; e.g. “Which is the banana? Can you see the banana? Show me the banana.”) and once for the unfamiliar named object (‘novel name trial’; “Which is the dax? Can you see the dax? Show me the dax.”). As young children display an intrinsic preference for novelty in referent selection tasks (e.g. Horst et al., 2011), familiar name trials were included to prevent participants from learning to simply choose the novel object and encourage them to examine every item. Indeed, effective fast mapping requires children to attend to known competitors in order to exclude them as referents for a novel word (Halberda, 2003). Novel name trials were designed to promote active learning of new word-object pairings. As the child already has names for the two familiar objects, they should apply the mutual exclusivity principle and assign the novel label to the unfamiliar named object.  
The order of trials was pseudo-randomised with the constraint that the same set of objects was never presented on consecutive trials and no more than two trials of the same type (familiar name or novel name) were experienced sequentially. Positioning of objects on the tray (left, middle, right) was pseudo-randomised across trials with the constraint that the requested object did not appear in the same location more than twice consecutively. The four novel words (dax, wug, yok, lep) were randomly allocated to the four unfamiliar named objects for each participant. 
Test object familiarisation. Test object familiarisation immediately followed the final referent selection trial. Children were familiarised with the as-yet unseen shape/colour match objects before their appearance in the subsequent test trials. The purpose of this stage was to minimise novelty and familiarity preferences, thus increasing the likelihood that children would select objects based on their memory of word-object mappings.  Each of the shape/colour match objects was paired with a previously seen named object belonging to a different set. Pairs of objects were presented on the left and right sections of the tray (approximately 48cm apart) and the experimenter instructed children to “look!”. The experimenter silently counted to six before removing the objects from view and presenting the next pair until all of the novel objects had been seen. Object pairings were fixed and children received one of two presentation orders. Positioning of named objects and shape/colour match objects to the left and right was randomised.
Delay. Immediately following test object familiarisation, the child played with the experimenter for 5 minutes. None of the familiar or unfamiliar objects used in the experiment were visible during this stage.
Retention and generalisation test trials. To re-engage children’s attention to the task, the experimenter selected 3 familiar objects at random and administered one warm-up trial as described above. Eight retention trials and eight generalisation trials immediately followed (see Figure 2 for an illustration of each trial type). Each novel word was tested on 2 retention trials and 2 generalisation trials. For retention trials, 3 named objects were presented in different sections of the tray (left, centre, right). The experimenter asked children to identify one of the objects (e.g. “Which is the dax? Can you see the dax? Show me the dax.”) and then slid the tray forward. The purpose of these trials was to assess children’s memory of the exact word-referent pairings that were experienced during the referent selection trials. For generalisation trials, children were presented with a differently coloured variant of a target named object (shape match), an object that matched the target named object on colour but not shape (colour match), plus a shape/colour match object for another set. Children were asked to identify which of these novel objects was a referent for the word paired with the target named object (e.g. “Which is the dax? Can you see the dax? Show me the dax”). The purpose of these trials was to assess whether children’s extension of labels to novel referents is systematically influenced by shape or colour. Importantly, all choice objects were of equal familiarity in both retention trials (all were named in referent selection trials) and generalisation trials (all were introduced during test object familiarisation and were differently coloured variants of a named object). 
To provide the necessary level of control when presenting stimuli, object groupings were fixed. Trials were administered in one of 8 possible orders (evenly allocated across participants in each sample). For each order, the experimenter alternated between retention and generalisation trials and never requested the same word on consecutive trials. Half of the orders started with retention trials and half started with generalisation trials. The positioning of object sets in the sequence of trials was systematically varied across orders. In the retention trials, the 4 named objects each served as a target on two trials and a foil on four trials. In the generalisation trials, each shape match object (a) corresponded with the requested word on 2 trials (i.e. acting as the target), (b) served as a colour match on 2 trials, and (c) served as a distracter on 2 trials. Positioning of objects on the tray (left, middle, right) was pseudo-randomised across trials with the criterion that the requested object did not appear in the same location more than twice consecutively.

Warm Up trial

Referent Selection
 trial

Retention test
 trial

Generalisation test
 trial

Example objects presented

Fig. 2 Example objects presented at each stage of Experiments 1 and 2. The target object is the middle object in the referent selection trial, the left object in the retention test trial, and the right object in the generalisation test trial.

Experiment 2: The effect of reinforcement on word learning in children with ASD and typical development
Materials
Stimuli were exactly the same familiar and unfamiliar objects as used in Experiment 1. The same tray was used to present stimuli, however, an “illuminating” function was used in the Non-social Reinforcement condition. Underneath each section of the tray was a light bulb that could be activated by a wired switch that was concealed on the experimenter’s side of the table. When switched on, the light bulb highlighted the object in the section above.
Procedure
The procedure was exactly as described for Experiment 1 with some adjustments to the referent selection trials of the word learning task (detailed below).
Social Reinforcement condition. Eight referent selection trials (four familiar name and four novel name) followed the warm-up trials as described for Experiment 1. The stimuli and format were as previously described, with one exception. After children’s response to each trial (whether correct or incorrect), the experimenter ostensively named the target object with supporting social cues (see Figure 3). They directed their gaze towards the location of the target (with accompanying head turn), pointed at the object with their index finger, and stated its name (e.g. “That is the [object name]!”). Importantly, the spatial location of the three objects in each trial did not change. The experimenter ensured that the participant was attending to the objects before providing social cues and naming. If the child was looking elsewhere, the experimenter re-captured their attention by excitedly saying their name and/or “look!”. 
Non-social Reinforcement condition. The Non-social Reinforcement condition differed from the Social Reinforcement condition in terms of how the experimenter named the target objects. After children’s responses, the experimenter discreetly flashed the light bulb underneath the target object’s tray compartment on and off four times (see Figure 3). While the light was flashing, the experimenter named the target object (e.g. “That is the [object name]!”). The experimenter looked straight at the child while the light was flashing and did not look at any of the objects in the tray (thus providing no social cues).  As described above, the experimenter ensured that the participant was looking at the objects before activating the light and naming.

Fig. 4 Examples of how reinforcement in referent selection trials was provided following children’s responses in the Social Reinforcement (top) and Non-social Reinforcement (bottom) conditions of Experiment 2.